Power scalable optical systems for generating, transporting, and delivering high power, high quality laser beams

ABSTRACT

Power scalable, rectangular, multi-mode, self-imaging, waveguide technologies are used with various combination of large aperture configurations,  20, 50, 80, 322, 324, 326, 328, 330, 332, 334, 336, 338 , Gaussian  360  and super-Gaussian  350  beam profiles, thermal management configurations  100 , flared  240  and tapered  161  waveguide shapes, axial or zig-zag light propagation paths, diffractive wall couplers  304, 306, 308, 310, 312, 314, 316, 318, 320  and phase controller  200 , flexibility  210 , phased arrays  450, 490 , beam combiners  530, 530 ′, and separators  344, 430 , and other features to generate, transport, and deliver high power laser beams.

CROSS-REFERENCES TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 10/117,445, filed on Apr. 5, 2002 now U.S. Pat. No. 7,042,631, whichis a continuation-in-part of U.S. patent application Ser. No.09/968,974, filed on Oct. 1, 2001 now U.S. Pat. No. 6,894,828, each ofwhich is incorporated herein by reference, and claims the benefit ofU.S. provisional application No. 60/259,681, filed on Jan. 4, 2001.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention under ContractNo. N68335-00-C-0486 between the United States Department of Defense andCoherent Technologies, Inc., and certain Air Force contracts.

TECHNICAL FIELD

This invention is related generally to laser beam generation,amplification, and delivery systems and, more specifically, to powerscalable optical systems for high power laser radar (ladar)applications, such as long-range targeting, imaging, and otherapplications, and includes light beam amplifiers, laser devices,waveguide technologies, and optical coupling, switching, and beamsteering technologies.

BACKGROUND OF THE INVENTION

Long-range laser radar (ladar) systems for ranging, imaging, tracking,and targeting applications need highly spatially and spectrally coherentcontinuous wave (CW) output or short pulses of high-power, neardiffraction limited, beams with high spatial coherency and neartransform limited spectral content for accuracy and long rangecapabilities. Typical platforms for such ladar systems includehigh-performance aircraft, spacecraft, weapons systems, or portableequipment in which space is limited, but power output, beam quality, andbeam control requirements are paramount. There are also many otherapplications for optical amplifiers, lasers, beam transport systems, andbeam input/output and switching controls that are efficient, low mass,and compact in size, yet can be scaled to high average as well as peakpowers while producing a high quality, near diffraction limited, beam.For example, laser beams are used extensively in industry for materialsprocessing, cutting, and drilling applications and in medical surgicalprocedures in which very narrowly focused, high intensity beams producesharper, cleaner cuts.

A common TEM₀₀ beam is one type of beam in which the light energy isspatially coherent (same phase across the thickness or cross-section ofthe beam) and is the lowest spatial mode of a laser. (Spatial mode incontext of spatial coherency refers to the degree to which the laser isspatially coherent and should not be confused with modes of lighttransmission or propagation in a waveguide, which are also discussedherein.) A TEM₀₀ beam has a Gaussian amplitude distribution and can befocused down to the smallest size—much more so than higher modes, thusconcentrating the light energy in the beam to a high intensity. A TEM₀₀beam can also be propagated for long distances with minimal spreading orexpansion of beam size. For many applications, therefore, it isdesirable to pack as much energy as possible into TEM₀₀ beams. Forexample, for cutting materials, packing more energy into a TEM₀₀ beammeans more power that can be focused to a very small spot to cut better,sharper, and cleaner than, a higher mode, e.g., TEM₀₁, or TEM₁₀, thathas less spatial coherency of the light energy in the beam.

For laser radar (ladar) detection of range (distance away), velocity(speed and direction of travel), and even shapes or images of objects,such as targets, from long distances away, high average power CW lasersor pulses of TEM₀₀ beams are preferred for minimizing power loss ofbeams propagated over such long distances due to beam spreading,scattering, and attenuation in the atmosphere. Further, to maximize thelikelihood that light reflected by the target back to the ladar receiverwill still be strong enough to be detected in the midst of all otherlight energy of similar wavelengths in the atmosphere (backgroundnoise), which also reaches the range detector, the launched light beamshould have a high level of energy. However, if there are severaltargets or objects close to each other, a long pulse will not allow therange detector to distinguish between light energy reflected from theseveral targets or objects, respectively, unless high bandwidthamplitude modulated (AM) or frequency modulated (FM) chirp is utilized.Such range discrimination, i.e., the minimum distance separating tworeflective surfaces that can be detected separately, is even morecritical in laser imaging applications in which the range detector mustbe able to discriminate between different reflecting surfaces of thesame object or target in order to determine its shape. Such imagingalong with range detection may be used, for example, to distinguishbetween an enemy tank and an adjacent house or to determine if anairplane has the shape of a commercial airliner or a military bomber.Therefore, to detect and image targets at the longest range (distanceaway), beams of short, high-power, pulses with near diffraction limited,spatial coherency and near transform limited spectral content are mosteffective, although use of high bandwidth AM, FM, or phase modulated(PM) light followed by an optimized matched filter receiver is also veryeffective in many applications.

Unfortunately, prior to this invention, typical adverse, non-linear andthermal effects, such as thermal self-focusing and self-phasemodulation, stress birefringence, stimulated Rayleigh, Raman, andBrillouin scattering, intermodel dispersion, and the like, limit theamount of power that can be produced by current state-of-the-artwaveguide resonators and amplifiers, such as those that use crystallinelaser materials in the form of bulk rods or slabs pumped with laserdiodes and non-crystalline materials such as glassy optical fibers.However, at high powers, it is difficult to achieve both excellentpump/mode matching with high pump absorption and diffraction-limitedbeam quality. Longitudinal pumping can result in excellent modematching, but it is limited in power due to the thermal stress fracturelimit, i.e., the medium will crack when it gets too hot [S. Tidwell etal., “Scaling CW Diode-End-Pumped Nd:YAG Lasers to high Average Powers,”IEEE J. Quantum Electron, vol. 28, 997 (1992)].

Another common problem in state-of-the-art bulk laser geometries priorto this invention is thermal management—both in the form of heatextraction and dissipation as well as optical distortion due to thermalgradients. The heat build-up results from absorption of high pump energyin a small volume of laser material, and active cooling in the form ofbulky heat exchangers or refrigeration systems is usually required toremove the heat. Such active cooling adds severely to the size, weight,and power requirements of the system. Thermal gradients in the lasermaterials are manifested in the forms of undesirable thermal lensing orself-focusing, due to thermally-induced birefringence, which alterspolarization of the light. See, for example, David Brown, “NonlinearThermal Distortions in YAG Rod Amplifiers”, IEEE J. Quantum Electron,vol. 34, 2383 (1998). Considerable research has been devoted tocompensation schemes for these adverse thermal effects. These problemsare significant, because there is typically power dependentbirefringence, which alters polarization, and bi-focusing, whichdegrades spatial and temporal coherence. See, James Sherman, “Thermalcompensation of a CW-pumped Nd:YAG laser”, Appl. Opt., vol. 37, 7789(1998). One technique that has been tried to alleviate this effect is touse extremely thin laser media (“thin disks”) such that thermal gradientis reduced and one-dimensional. See U. Branch et al., “Multiwattdiode-pumped Yb:YAG thin disk laser continuously tunable between 1018and 1053 nm”, Opt. Lett., vol. 20, 713 (1995). However, operation inquasi-three-level laser material (Yb, Er, Tm, Ho) severely exacerbatesthe thermal problem, since it requires much higher pumping to reachthreshold and/or refrigerated cooling to depopulate the thermal laserlevel. Consequently, there has not been any real solution to the thermalproblems when scaling bulk laser materials to high power levels.

Optical fiber lasers and amplifiers overcome some of the thermalproblems of bulk laser crystal materials by greatly increasing thelength of the gain medium and providing mode confinement, i.e., limitingthe size of the fiber core diameter so that it can only propagate thelowest order eigenmode, (so-called “single-mode fibers”). There areseveral benefits to this approach, including: (i) the long interactionlength between the pump light and the laser beam lead to high gain andefficient operation, even in 3-level lasers in which the terminal laserlevel is thermally populated; (ii) Heat is distributed over a longerlength of laser medium with a larger surface area, so the heat can bedissipated with passive conductive cooling to the atmosphere or to aheat sink; (iii) Operation can be restricted to a single transversemode, which preserves a TEM₀₀ spatial coherence and Gaussian intensityprofile for the beam focusability and beam propagation with minimal beamspreading advantages as described above; (iv) The flexible nature of theoptical fibers allows compact and novel optical designs; (v) The opticalfibers can be directly coupled to other passive or active waveguides formodular functionality; and (vi) Fabrication is suited to large-scaleproduction, which reduces costs. However, power scaling, i.e., scalingup to higher power levels, in such single-mode optical fibers isrestricted by inability to make efficient coupling of pump light energyinto the optical fiber and by the minute, single-mode core, (10–30 μmdiameter) holey or photonic fibers and core cluster fibers, which canonly handle so much light energy without overheating, distortion, orsuffering catastrophic facet (coupling surface) failure.

This limitation of fiber lasers and amplifiers has been partly overcomeby use of a double-clad fiber structure in which the small-diameter,single-mode core is surrounded by an inner cladding region, which, inturn, is surrounded by an outer cladding region. The inner claddingregion has a larger numerical aperture than the core, thus can acceptmore pump light energy in more modes. Therefore, the pump light isoptically confined to both the core and inner cladding regions together,while the optical beam is confined to the core alone. However, drawbacksof such double-clad fiber designs for laser resonators and amplifiersinclude: (i) The pump light energy, while introduced into, and confinedby, the core and inner cladding together, is absorbed only in the coreregion so that the effective absorption coefficient is reduced byapproximately the ration of the core area to the inner cladding area;(ii) The inner cladding size is still very small, even though largerthan the core, so that coupling of a laser diode array into the innercladding region is still quite difficult and not very efficient; and(iii) The outer cladding region must be made with a much lower index ofrefraction than the inner cladding for optical confinement of the pumplight to the inner region, and such lower index of refraction materialsare often polymers (plastic), which are much more susceptible to damagethan glass, especially from heat.

Essentially, the single-mode or large effective area core diameter ofoptical fibers is so small (10–30 μm, which is equivalent to 7.8×10⁻⁷cm² in cross-sectional area) that a 10 μJ (micro joule) pulse of lighthas a fluence (energy per unit area) greater than 13 J/cm² (joules persquare centimeter), which is close to the damage threshold of the fiber.Larger core diameter can handle more energy, of course, so that a 10 μJpulse of light would not be so close to the damage threshold, but largercore diameters result in undesirable eigenmode mixing and resulting lossof polarization, spatial coherence, and temporal coherence, which aresignificant beam degradations that reduce usability and effectiveness ofthe beam and should be avoided. Some complex-design, large-area,multi-mode fibers have been reported with reduced mode-mixing and pulseenergies up to 500 μJ with M²<1.2, (M² is a measure of divergencerelative to diffraction limit and M²=1 is diffraction limited) have beenreported [see, e.g., H. Offerhaus et al., “High-energy single-transversemode Q-switched fiber laser based on multimode large-mode-areaerbium-doped fiber”, Opt. Lett., vol. 23 (1998)], but no trulysingle-mode (LP₀₁) fiber design has been able to break thenanosecond-class, short pulse, 1 mJ (1,000 μJ) barrier, whilemaintaining spectral and spatial coherence with short temporal pulsewidths.

In many applications, including those addressed by this invention,production and amplification of high-power, high quality laser and otherlight beams is only part of the problem. Transporting such high-power,high-quality beams to points of application, such as the industrialcutting and materials processing, medical, laser radar ranging, imaging,and tracking applications mentioned above, can also present heretoforeunsolved problems. For example, in the laser radar (ladar) systemdescribed in U.S. Pat. No. 5,835,199, which is incorporated herein byreference, a high-power laser beam is produced for launching fromairplanes or other platforms for ranging, imaging, and tracking objectsor targets as much as twenty miles away or more. In an airplane, themost effective launch point for such high-power beams may be in the nosecone or in other extremities of the airplane where space is tight andwhere many electronic and other kinds of equipment also have to fit.Consequently, it is often not possible to place high power laser beamproduction and amplifying equipment at the most effective launchlocations in the airplane. Therefore, it would be very beneficial tohave some way of transporting high power laser beams from some otherlocation in the airplane to one or more launch points in the nose cone,fuselage floor, wings, tail, or other structures without degrading beampower, quality, polarization, and the like, and to have an effective wayof directing or steering such high power beams at such remote launchpoints for the best overall ranging, targeting, or imaging results.

Similar beam transport capabilities would also be beneficial inindustrial, medical, imaging, directed energy, and other applications ofhigh power laser and other light beams, where space is limited or whereit would just be more convenient to place a high powered, high qualitybeam without all the associated beam production and/or amplificationequipment.

Yet, transport of high power, high quality laser beams withoutdegradation of beam power, quality, temporal and spatial coherence,polarization, and the like presents serious problems with many of thesame kinds of obstacles as described above for the beam production andamplification. For example, single-mode waveguides, such as single-modeoptical fibers, can maintain beam quality, but their very small size forsingle-mode operation severely limits power transport capabilities.Industrial medical, and even imaging applications would benefit fromcontinuous wave (cw) output power of 100 watts or more, while evenhigher power laser applications, such as Q-switched or pulsed lasers,may have output power in the megawatt range, such as 10 megawatts orgreater. Single-mode waveguides, including fibers, are simply unable tohandle that kind of optical power or light energy.

Multi-mode fibers and waveguides are larger than single-mode fibers andwaveguides, thus can transport more power, but they do not maintainspatial coherence, polarization, and the like, because of multi-modeinterference and other reasons mentioned above. Free-space lighttransport has its own problems, not the least of which is that the lightpaths have to be unobstructed and alignment and stability problems innon-laboratory environments are extremely difficult to overcome and areoften insurmountable.

Techniques have been previously developed to actively compensate forfinite length circular fiber spatial mode deficiencies, including SBSphase conjugation, but these techniques are limited in scope to narrowspectral line width lasers to match the SBS gain bandwidth, enoughoptical power to provide the nonlinear drive field required, andwavefronts that are not fully reconstructed by phase conjugation.Furthermore, and as previously mentioned, it may be desirable in manywaveguide applications to maintain polarization. In circular fibers witha uniform index-of-refraction in both the core and cladding,polarization may not be maintained. To preserve polarization, specialpolarization-maintaining fiber designs may be required which essentiallycreate an asymmetric index difference in orthogonal directions. If thisindex profile is disturbed, potentially as a result of high poweroperation, the polarization integrity may drift or be lost.

Beam quality issues may arise, for example, related to mode mixing aspreviously described, or with regard to “bend, buckle and twist” of thewaveguide and potentially resulting modification of at least spatialcoherence, wherein, for example, a twist of the waveguide could resultin beam formation equivalent to a negative lens, and a bend in thewaveguide may result in beam formation potentially equivalent to apositive lens. Such applications of waveguide technology have not beenadequately addressed in the past attempts previously described or inother previous beam transport technologies.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of this invention to provide anoptical system that is scalable to high average and peak power laserbeam production, amplification, and control capabilities for ladar,industrial, medical, direct energy (DE) weapons, and other applications.

Another general object of this invention is to provide high power,diffraction limited, laser beams and amplifiers and waveguide systemsthat are capable of delivering such high power laser beams to one ormore points of application without significant degradation of beamquality.

A more specific object of this invention is to provide an optical systemfor a high power ladar application, including flexible beam transportwaveguides with remote controllable wall coupling and beam directingcapabilities that are suitable for volume-limited platforms.

Another object of the present invention is to provide an optic amplifieror laser resonator that can be operated at much higher optical powersand Q-switched pulse energies than is possible in single-mode waveguidesor fibers, while: (i) also having many of the excellent benefits ofsingle-mode waveguide or fiber amplifiers and lasers, includingpreservation of polarization and capability of maintaining a TEM₀₀(Gaussian) beam profile (or any other desirable waveform); and (ii)avoiding undesirable nonlinear effects that are inherent in single-modewaveguide or fiber amplifiers and lasers operated at higher intensities,such as stress birefringence and self-phase modulation.

Another general object of the present invention is to provide anapparatus and method for producing high power continuous wave (CW) laserbeams with near arbitrary spectral coherence that are capable ofmaintaining a diffraction limited wavefront for sharp focusingcapabilities in industrial materials processing and fabrication, medicalsurgical applications, and any other application in which sharp focusingof high power beams would be beneficial.

Another general object of the present invention is to provide anapparatus and method for producing high power, pulsed laser beams forpacking higher energy into shorter pulses for longer ranging and higherresolution target acquisition and imaging applications.

Another object of the present invention is to provide a high poweroptical amplifier or laser resonator that has efficient heat dissipationand that can be mated easily and effectively to one or more heat sinks.

Still another object of the present invention is to provide a highpower, yet compact and lightweight optical amplifier or laser resonator.

A further specific object of the present invention is to provide a highpulse energy or high average power quasi-continuous-wave (QCW) or highrepetition rate macro-pulse laser which can be frequency converted toany band, including Band IV, for defense infra-red countermeasureapplications.

A still further specific object of the present invention is to provide afew Hz to multi-GHz-class repetition rate laser source that can bepumped efficiently quasi-cw or low cost, continuous wave, diode laserand which can be frequency converted to eye-safe wavelengths for targetidentification and ranging and unconventional active imagingarchitectures.

Another specific object of the present invention is to provide a methodof frequency shifting and/or amplifying a guided wave in a self-imagingwaveguide.

Another specific object of the invention is to provide a method forstabilizing an internal propagating mode by compensating linear ornon-linear dispersion terms (e.g., achromatization and solutionformation by using linear or non-linear, e.g., intensity dependent,index of refraction media in the waveguide core.

A more specific object of the present invention is to mitigateundesirable effects of thermal focusing in high power optical amplifiersso that one waveguide design can be used for various average powers andpulse formats for a variety of applications, thereby making it feasiblefor one design to span many applications.

Another object is to provide spectral and spatial coherence control thatis adequate for use of waveguides, both hollow and solid, e.g.,dielectric, beam transport, and especially directed energy applicationssuch as high power weapon applications as elements of an optical phasedarray, which is typically defined as near diffraction limited ensemblewavefronts with optical phase control to less than one-tenth of a wave.

Another specific object of the present invention is to provide low costand efficient coupling of laser diode arrays into an active opticalamplifier medium or laser resonator for high conversion efficiency.

Still another specific object of the present invention is to provide ahigh power scalable optical amplifier or laser resonator that hasexcellent pump light to beam overlap and high energy extractionefficiency.

Yet another specific object of the present invention is to provide ahigh power optical amplifier or laser resonator in which the activemedium has non-deleterious thermal gradients while pump light energy isdistributed over a large volume and surface area.

Another general object of the present invention is to provide guidedwave systems and beam transport providing desirable capability for highpower applications. It is a goal of the present invention, therefore, toprovide guided wave systems, beam transports, or waveguides that providefor particular beam types, particular output power requirements ofdesirable waveguide and beam transport applications, and desirablelevels of beam quality.

Yet another object of the present invention is to provide self-imagingguided wave systems and beam transport while achieving desirable levelsof beam quality and capability for high power applications. It is onegoal of the present invention, therefore, to provide guided wave systemsand beam transport providing desirable polarization and spatial,spectral, and temporal coherence characteristics. Furthermore, it is agoal of the present invention to provide self-imaging guided wavesystems and beam transport while minimizing or eliminating potentialoptical damage to the waveguide and nonlinear optical effects.

Still another object of the present invention is to provide guided wavesystems and beam transport that may be applicable, and potentiallycomprise, one or more potentially desirable beam transport features. Acorresponding goal, therefore, is to provide guided wave systems andbeam transport that may be applicable, and potentially comprise, one ormore features such as synthetic aperture, distributed aperture, beamforming, beam steering, beam combining, power sampling, power combiningand power splitting, among other features.

A further object of the present invention is to provide guided wavesystems and beam transport that may be applicable to one or more fields,including telemetry, aeronautical and space applications, directedenergy systems, object imaging systems, object positioning and trackingsystems, detection systems, fiber optics, machine fabrication, andmedical systems, among others.

Yet another object of the present invention is to provide guided wavesystems and beam transport that are adapted to aeronauticalapplications, and aircraft applications. A goal of the presentinvention, therefore, is to provide guided wave systems and beamtransport comprising a configuration particularly directed to directedenergy systems, object imaging systems, object positioning and trackingsystems, and detection systems for aircraft and other aeronautical andspace applications, while maintaining desirable beam quality and highpower characteristics. A further related goal is to provide for “bend,buckle and twist” characteristics of the guided wave systems and beamtransport, while either maintaining or providing for resultingmodification of spatial coherence and applications thereof.

Additional objects, advantages, and novel features of the invention areset forth in part in the description that follows and will becomeapparent to those skilled in the art upon examination and understandingof the following description and figures or may be learned by thepractice of the invention. Further, the objects and advantages of theinvention may be realized and attained by means of the instrumentalitiesand in combinations particularly pointed out in the appended claims.

To achieve the foregoing and other objects and in accordance with thepurposes of the present invention, as broadly embodied and describedherein, the high power optical amplifier and/or laser resonator of thisinvention may comprise an optic amplifier for a laser beam and/or alaser resonator that includes a solid-state, multi-mode, self-imagingrectangular, waveguide with a core comprising a solid gain medium, whichcan be excited or pumped with energy and can impart such energy to alight beam propagating through such solid waveguide core. The amplifieror laser resonator includes optical components that focus or otherwiseprovide a desired beam spatial profile, such as super-Gaussian, on aface or aperture of the solid, rectangular waveguide, and the waveguidelength coincides with a waveguide self-imaging period (WSIP) of therectangular waveguide or some non-zero, ¼-period or integer multiplethereof, in order to produce that same spatial profile in an amplifiedoutput beam. The multi-mode, rectangular, waveguide core may be unclad,partially clad in sandwich cladding, or fully clad in or fully envelopedby cladding. Rectangular-shaped cladding is particularly beneficial forheat sink mountings, electrical excitation, and optical pumping withelongated, stacked laser diodes, although rectangular waveguide coresclad in optical fibers with circular, oval, or other cross-sectionalshapes are also useful in various applications of this invention. Azig-zag waveguide optical path, which increases effective energyextraction in a smaller overall length, is particularly adaptable toone-dimensional, or quasi-one-dimensional, solid-state, multi-modewaveguide cores according to this invention.

Embodiments of the invention may also comprise passive, hollow anddielectric core multi-mode, guided wave, beam transport systems.Embodiments may include rectangular or square cross-section waveguides,and preferably maintaining spatial profile of an input beam, such as aGaussian or super-Gaussian beam, through the self-imaging period of thewaveguide. Additional aspects of the present invention may be providedeither separately or in conjunction with the self-imaging guide of thepresent invention; for example, transport, amplification,phase/frequency control or modulation, deflection, conversion, syntheticaperture, distributed aperture, beam forming, beam steering, beamcombining, power sampling, power combining and power splitting, amongother features.

To further achieve the foregoing and other objects and, in accordancewith purposes of the present invention, and as broadly embodied anddescribed herein, a method of providing a high power, diffractionlimited laser beam to a desired application comprises producing a highpower beam with a desired spatial profile, coupling the beam into anelongated, multi-mode, self-imaging, transport waveguide that has aleast one output aperture positioned at a self-imaging plane, andcoupling the beam out of the output aperture for a desired application.According to one aspect of the invention, a high power laser beam can beproduced by an advantageous combination of a rectangular (preferablyone-dimensional), multi-mode, self-imaging, solid core, amplifierwaveguide with a heat sink in thermally conductive contact with at leastone, and preferably two, of the large aspect (waveguiding) sides of thewaveguide. While this combination works with any desired beam profile,another advantageous addition to the combination, thus inventivefeature, is to condition the beam to have a super-Gaussian (preferably alow order super-Gaussian) profile, which can enhance scalability to highpower levels while delivering high quality, diffraction limited, beams.At the same time, the self-imaging feature of the waveguide, thusability to use larger aperture waveguides, also facilitates scaling tohigh power levels, even though such larger aperture waveguides aremulti-mode. Scaling to high power levels is also facilitated by theflat, thin, elongated, rectangular, cross-sectional profile of arectangular (especially one-dimensional), multi-mode, self-imaging,waveguide core, which provides efficient heat dissipation from the core.Delivering the high power laser beam through an elongated, multi-mode,rectangular, self-imaging, transport waveguide (preferably, but notnecessarily, hollow) can include twisting and/or bending the transportwaveguide to route the outlet aperture to a desired position. Changingindex of refraction of a window, such as a diffraction grating, liquidcrystal, or other material at a re-imaging plane provides an outletaperture for some or all of the beam with the desired spatial profile. Agrating (preferably comprising numerous spaced-apart strips of theliquid crystal material) can direct or steer the beam. The output beamof one transport waveguide can be coupled into another multi-mode,rectangular, self-imaging, transport waveguide, or it can be combinedwith another, phase-matched, beam to produce an even higher power beamwith the desired spatial profile.

Embodiments of the present invention may provide a self-imaging,multimode waveguide as disclosed and claimed herein, and self-imagingguided wave systems and beam transport. Embodiments of the presentinvention may further provide a method of self-imaging, multimode beamtransport, as disclosed and claimed herein, and other self-imaging waveguidance techniques. Other embodiments of the present invention may alsobe disclosed and claimed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the preferred embodiments of the presentinvention, and together with the descriptions serve to explain theprinciples and enabling embodiments of the invention.

In the Drawings:

FIG. 1 is a diagrammatic, perspective view of an airplane equipped witha high power optical system of this invention for ladar ranging,targeting, or imaging applications;

FIG. 2 is an example super-Gaussian amplitude spatial profile;

FIG. 3 is an example TEM₀₀ Gaussian amplitude spatial profile forcomparison to the super-Gaussian profile of FIG. 2;

FIG. 4 is a diagrammatic, isometric view of a single-pass high poweroptical amplifier of this invention with a portion of the rectangular,multi-mode, self-imaging, waveguide cut away to reveal the core andcladding structure;

FIG. 5 is a transverse cross-section view of an example largeaspect-ratio rectangular or “quasi-one-dimensional” waveguide takensubstantially along section line 5—5 of FIG. 4;

FIG. 6 is a transverse cross-section view of an example rectangular,multi-mode waveguide similar to FIG. 5, but with the aspect ratiosmaller or “two-dimensional” so that the sides are also effectivelyoptically confined by the cladding;

FIG. 7 is a transverse cross-section view similar to FIG. 5, butillustrating an example square waveguide core, which is a special caseof a rectangular waveguide;

FIG. 8 is a longitudinal cross-sectional view of an example rectangular,multi-mode, self-imaging waveguide taken along section line 8—8 of FIG.4 to illustrate evanescent field or propagation effects on length ofwaveguide self-imaging period (WSIP);

FIG. 9 is a diagrammatic, isometric view of a double-pass, high-poweroptical amplifier of this invention with a reflective end face;

FIG. 10 is a diagrammatic, isometric view of a double-pass, high-poweroptical amplifier similar to FIG. 9, but with an external end reflector;

FIG. 11 is a diagrammatic, isometric view of a high-power laserresonator according to this invention;

FIG. 12 is a diagrammatic view of a non-clad, multi-mode, rectangularwaveguide in an amplifier laser resonator embodiment of this invention;

FIG. 13 is a diagrammatic, isometric view of a side-pumped andend-pumped high-power optical amplifier according to this inventionmounted on a heat sink with phantom lines illustrating optionaldouble-faced heat sink couplings;

FIG. 14 illustrates a multi-mode, rectangular, waveguide clad in anoptical fiber embodiment for use in an optic amplifier, laser resonator,beam transport, or other application of this invention;

FIG. 15 illustrates a multi-mode, rectangular waveguide that isdouble-clad in an optical fiber embodiment that is particularly usefulfor providing pump light energy through the intermediate cladding layerto the multi-mode, rectangular waveguide core in amplifier or laserresonator applications of this invention;

FIG. 16 is an isometric, diagrammatic view of a multi-mode, rectangularwaveguide embodiment in which a beam is propagated in a zig-zag path toincrease amplification and energy extraction efficiency;

FIG. 17 is an isometric, diagrammatic view of a variation of the zig-zagoptical path waveguide embodiment of FIG. 16, but with tapered sideedges for higher zig-zag path density near one end and returnpropagation back through the waveguide;

FIG. 18 is a cross-sectional view of a zig-zag, double-pass, beam pathin a rectangular, multi-mode, self-imaging waveguide amplifier accordingto this invention;

FIG. 19 is a cross-sectional view of a diffractive coupler, e.g., liquidcrystal modulator, for wavelength and phase control on a rectangular,multi-mode, self-imaging, waveguide amplifier or laser resonatoraccording to this invention;

FIG. 20 is an isometric view of an amplifier embodiment of thisinvention that is flared in the non-imaging (transverse) direction;

FIG. 21 is an enlarged, perspective view of portions of the high-powerbeam generator and transport system of FIG. 1;

FIG. 22 is an enlarged, perspective view of a high power beam transportwaveguide used as part of the invention;

FIG. 23 is a longitudinal cross-section of a perspective view of adiffractive coupler, e.g., liquid crystal, sidewall launch aperture,sometimes called a wall out-coupler, mounted in a rectangular,multi-mode, self-imaging, waveguides for high power beam launchingaccording to this invention;

FIGS. 24 a–c are cross-sectional views illustrating diagrammatically aliquid crystal window grating for beam directing or steering accordingto this invention;

FIG. 25 is a longitudinal cross-section view of a diffractive coupler,e.g., liquid crystal, aperture/switch for coupling light energy from onerectangular, multi-mode, self-imaging, waveguide to another according tothis invention;

FIG. 26 is a longitudinal cross-section of another liquid crystalcoupler/switch for rectangular, multi-mode, self-imaging, waveguidesaccording to this invention;

FIG. 27 is a perspective view of a stacked, phase-matched, array ofrectangular, multi-mode, self-imaging, waveguides of this invention;

FIG. 28 is a fiber embodiment of a phase-matched beam transport array ofrectangular, self-imaging, waveguides of this invention;

FIG. 29 is a perspective, underside view of an array of rectangular,multi-mode, self-imaging, waveguides with multiple sidewall launchapertures for synthesizing a high power beam at a point according tothis invention;

FIG. 30 is a perspective view of a high power beam combiner according tothis invention; and

FIG. 31 is a perspective view of another variation of a high power beamcombiner according to this invention.

DETAILED DESCRIPTION OF THE INVENTION

A high power optical system 300 according to this invention isillustrated in FIG. 1 as applied, for example, to laser radar (ladar)ranging, targeting, or imaging applications in a high performanceairplane A. As shown in FIG. 1, a ladar beam generator 302 is showndiagrammatically as mounted in an accessible location, such as aboutmid-fuselage near an access door, while a plurality of individuallycontrollable beam launch apertures 304, 306, 308, 310, 312, 314, 316,318, 320 are positioned at strategic, remote extremities or locations ofthe airplane A. The beam launch apertures 304, 306, 308, 310, 312, 314,316, 318, 320 are connected by large aperture, multi-mode, self-imagingwaveguides 322, 324, 326, 328, 330, 332, 334, 336, 338 to the ladar beamgenerator 302, so that the high power ladar beams, such as beam 340, canbe directed to targets 342 or other objects to be ranged, targeted, orimaged. Back-scattered wavefronts 341′ reflected or scattered from thetarget 342 can be coupled back into the transport waveguide 328 viaaperture 306 or into any other aperture and waveguide for transport totarget imaging or analysis equipment (not part of this invention). Aswill be described in more detail below, the launch apertures 304, 306,308, 310, 312, 314, 316, 318, 320 can be controlled individually to turnthe beams 342 on and off, and even to direct or steer them in relationto the airplane A. As will also be described below, wall couplers, suchas coupler 344, can connect adjacent waveguides optically to each otheroptically in a remote controllable manner, as will be described in moredetail below.

Of course, the high power optical system 300 can be mounted and used onmany different platforms other than an airplane A, can be configured ina variety of ways, and can be put to many uses other than targeting,ranging, and imaging. However, the airplane A platform mounting of thehigh-power optical system 300 in FIG. 1 is exemplary of importantfeatures and benefits of this invention, which will be described in moredetail below.

For weapons-class applications, the high power optical system 300 canproduce and deliver high power, continuous wave (CW), or Q-switched(pulsed), diffraction limited, laser beams, such as beam 340 in FIG. 1,with peak power in the range of 5–20 megawatts or more. Other highpower, weapons-class applications of the power scalable optical systemof this invention may have average power operations capabilities in therange of 1–3 megawatts or more. For example, 0.1 Joules per 10nanoseconds is 10⁷ watts. High power, continuous wave (CW), laseroperating ranges of the optical systems of the present invention formedical, industrial, imaging, communications, and other applications canbe scaled up to average power levels of 10–20 kilowatts or even 100kilowatts or more, with high quality, spatially and temporally coherent,polarized laser beams for optimal focusing and power usage.

Because a primary purpose of this invention is to provide laseramplifier, resonator, beam transport, and beam launching systems thatare scalable to high power levels for such uses as those described aboveand for other uses, it is helpful to provide some definition of highpower as used in describing this invention. High average power as usedherein in relation to waveguides for spatially coherent, continuous wave(CW), laser beam means about 10–100 watts per waveguide. Very highaverage power as used herein in relation to waveguides means a spatiallycoherent, continuous wave (CW), laser beam of about 100–1,000 watts ormore per waveguide. High peak power as used herein in relation towaveguides means spatially coherent, pulsed laser beam with peak powerin a range of about 1–10 megawatts per waveguide. Very high peak poweras used herein in relation to waveguides means spatially coherent,pulsed laser beam with peak power in a range of about 100–1,000megawatts or more per waveguide. High power, as used herein, means highaverage power, very high average power, high peak power, and/or veryhigh peak power, as defined above. Of course, multiple waveguides can bestacked into arrays, as discussed further below, to handle even higherpower levels than single waveguides. For example, stacked arrayscomprising a plurality of multi-mode, self-imaging waveguides aredescribed below. In general, the power handling capability of a largeaperture, multi-mode, self-imaging waveguide in this invention islimited only by thresholds of optical damage or thermal loading of thewalls, which can be actively cooled according to this invention, or byoptically induced breakdown of air or gas medium in the waveguide.

A combination of rectangular, multi-mode, self-imaging, waveguidetechnology with large aperture beam input and output and super-Gaussianbeam profiles enables power-scaling optical systems, according to thisinvention, up to and beyond the high power ranges described above forproduction and/or delivery of high power, spatially coherent,diffraction limited laser beams, as will be described in more detailbelow. While some principles of rectangular, self-imaging, technology,including its use in image transmissions, laser resonators, andamplifiers, are well-known, as in, for example, U.S. Pat. No. 3,832,029issued to O. Bryndahl, U.S. Pat. No. 4,087,159 issued to R. Ulrich, andU.S. Pat. No. 5,684,820 issued to R. Jenkins et al., applications andadaptations of such technologies to high power optical systems accordingto this invention are new.

An example super-Gaussian beam amplitude profile 350 and correspondingphase front 352 is illustrated in FIG. 2, as compared to a common TEM₀₀Gaussian beam amplitude spatial profile 360, and corresponding wavefront 362 illustrated in FIG. 3. The Gaussian beam amplitude spatialprofile 360 is typical of any diametric cross section of the beam and isa function of spherical wave or phase fronts 362. As explained above, aTEM₀₀ Gaussian beam is spatially coherent, i.e., the same phase, acrossany transverse cross-section of the beam, and super-Gaussian beams canalso have that attribute. However, super-Gaussian beams have differentintensity, or amplitude, spatial profiles than Gaussian, which canenable super-Gaussian beams to carry more power than Gaussian beams ofthe same cross-sectional size and shape.

In general, Gaussian, as well as super-Gaussian, beams can have circulardistribution, square distribution, elliptical distribution, orrectangular distribution of the light energy across a transversecross-section of the beam. The light intensity or amplitude spatialprofile, I (x,y), of such beams can generally be described as follows,where x is the x-direction of the waveguide, y is the orthogonal spaceaxis, and W_(x) is the width of the waist in the x-direction, W_(y) isthe width of the waist in the y-direction, and W_(o) is the diameter ofthe waist in the circular distribution case:

$\begin{matrix}{{{{Circular}\mspace{14mu}{distribution}\mspace{14mu}{I\left( {x,y} \right)}} \propto {\exp\left\lbrack {- \left\lbrack \frac{x^{2} + y^{2}}{W_{0}^{2}} \right\rbrack^{Sgxp}} \right\rbrack}},} & (1)\end{matrix}$

$\begin{matrix}{{{{Square}\mspace{14mu}{distribution}\mspace{14mu}{I\left( {x,y} \right)}} \propto {\exp\left\lbrack {{- \left\lbrack \frac{x^{2}}{W_{x}^{2}} \right\rbrack^{Sgxp}} - \left\lbrack \frac{y^{2}}{W_{x}^{2}} \right\rbrack^{Sgxp}} \right\rbrack}},} & (2)\end{matrix}$

$\begin{matrix}{{{{Elliptical}\mspace{14mu}{distribution}\mspace{14mu}{I\left( {x,y} \right)}} \propto {\exp\left\lbrack {- \left\lbrack {\frac{x^{2}}{W_{x}^{2}} + \frac{y^{2}}{W_{x}^{2}}} \right\rbrack^{Sgxp}} \right\rbrack}},} & (3)\end{matrix}$

$\begin{matrix}{{{Rectangular}\mspace{14mu}{distribution}\mspace{14mu}{I\left( {x,y} \right)}} \propto {{\exp\left\lbrack {{- \left\lbrack \frac{x^{2}}{W_{x}^{2}} \right\rbrack^{Sgxp}} - \left\lbrack \frac{y^{2}}{W_{x}^{2}} \right\rbrack^{Sgyp}} \right\rbrack}.}} & (4)\end{matrix}$In the above formulae (1)–(4), Sgxp is the super-Gaussian order in thex-direction, and Sgyp is the super-Gaussian order in the y-direction,and implicit are variations of the elliptical super-Gaussian to possessdifferent orders in the x and y directions.

If Sgxp=1 and Sgyp=1; then one obtains circular, square, elliptical, orrectangular Gaussians from the above formulae (1)–(4).

A super-Gaussian beam can have circular, square, elliptical, orrectangular light energy distributions described by formulae (1), (2),(3), or (4) above, and Sgxp can, but does not necessarily have to, equalSgyp, but either Sgxp or Sgyp, or both, is greater than one. As can beseen from a comparison of the Gaussian intensity profile 360 in FIG. 3with the super-Gaussian profile 350 in FIG. 2, the super-Gaussian beam350 has potentially more energy in a cross-section than the Gaussianbeam 360. Super-Gaussian beams also provide better energy extraction andhigher power as well as more uniform target illumination than Gaussianbeams. Lower order super-Gaussian beams also retain spectral and spatialcoherence across the cross-section, thus are diffraction limited, moreefficient, less chirped, and more desirable than even TEM₀₀ Gaussianbeams for many applications, such as, for example, coherent laser radarand phased radar operations, according to this invention. Spatialintensity variations in higher order, super-Gaussian beams can lead tospatial ringing and nonlinearly induced differences in complex index ofrefraction and, therefore, to distortion of spectral coherence.Similarly, variations of the intensity across a Gaussian beam lead tointensity dependent index of refraction changes, which impact chirp andwavefront quality. At some point, depending on the specific dimensionsof the self-imaging waveguide aperture, such propagation distortionswill become less acceptable. Because of such aperture size dependenceand nonlinearly induced differences, it is difficult to quantify aspecific line of demarcation between desirable, lower ordersuper-Gaussian beams and less desirable, higher order super-Gaussianbeams, in which the propagation distortions become unacceptable.However, in general, super-Gaussian beams in which both Sgxp and Sgypare less than 7 can be considered lower order super-Gaussian andfunction in applications of this invention without excessive distortionof either spectral coherence or spatial coherence.

Super-Gaussian beams, including, but not limited to, lower ordersuper-Gaussian beams according to these parameters, can be created in avariety of ways, including, but not limited to, amplitude and/or phasemodifications to another beam format that is being injected into anoptical amplifier or laser resonator via binary optics, such as phaseplates, or any other optical component to accomplish such modifications,as would be understood by, and would be well within the capabilities of,persons skilled in the art, once they understand the principles of thisinvention. However, to illustrate such a phase modification generally, astep relief, phase modification plate 351 in transmissive mode isillustrated diagrammatically in FIG. 2 as it may be used to modify aspherical phase front 353 to produce a modified phase front 352 in orderto produce a super-Gaussian amplitude profile 350. While a phasemodification plate can be stepped relief and transmissive, as shown inFIG. 2, it could also be stepped relief and reflective, continuousrelief and transmissive, or continuous relief and reflective. Otherexamples of suitable optical components for such amplitude and/or phasemodifications of beams may include spatial light modulators, Demanngratings, modans, kineforms, and many others well-known to personsskilled in the art, all of which would work for purposes of thisinvention.

An optic amplifier 10 based on a rectangular, multi-mode, self-imaging,waveguide 20 with large, rectangular, input and output apertures oropposite end faces 26, 28 that can support a super-Gaussian beam 14according to this invention is shown diagrammatically in FIG. 4 in asingle-pass embodiment for illustrating basic features, while adouble-pass embodiment is described below and shown in a subsequentillustration. An important feature of this invention is the use of arectangular, multi-mode, self-imaging waveguide 20, with large input andoutput apertures 26, 28, as the gain medium for the high-power, opticalamplifier 10, so that the amplifier 10 can deliver a high power beamwith a wide range of spatial profile modes up to the spatial frequencylimit of the input aperture as defined by its optical transfer function,such as the highly desirable super-Gaussian, diffraction limited, beamdescribed above or a conventional Gaussian beam, without being confinedin power through-put by the constraints of a single-mode waveguide.

More specifically, the internal dimensions of a multi-mode wave guidecore, including the smaller dimension width or thickness “a” (see FIG.5), are large enough to support multiple eigenmodes of lightpropagation, as opposed to being so narrow as to support only one lightpropagation eigenmode as in a so-called single-mode waveguide. Any inputbeam 12 of a particular wavelength, including, for example, asuper-Gaussian beam, will quickly break up into as many eigenmodes ofpropagation as allowed for that wavelength by the multi-mode waveguide20 size, shape, and numerical aperture. Accordingly, for purposes ofthis invention, a multi-mode waveguide is one that does not restrictlight propagation to only one mode in a dimension. Thus, a single-modewaveguide is not considered to be included in the scope of the termmulti-mode.

In general, for multi-mode waveguides (not including multi-mode,self-imaging waveguides), each light propagation mode has a differentpath through the waveguide 20 and travels a different distance from theentrance face 26 to the exit face 28 than other modes, so the multiplemodes mix and interfere with each other, which degrades beam quality andquickly destroys the spatial coherence and super-Gaussian profile or anyother amplitude profile or distribution of the entering beam 12.Single-mode waveguides do not suffer from such interference andresulting degradation of beam quality, thus are capable of preservingand delivering a high quality, spatially coherent, beam profiles attheir output apertures, but single-mode waveguides cannot handle thescaled up power levels of the larger, multi-mode waveguides.

However, according to a well-known re-imaging characteristic ofrectangular and other parallelepiped waveguides, the opticalinterference patterns in the waveguide re-phase and reconstruct an inputimage at periodic distances along the waveguide. The distance for suchperiodic re-imaging, sometimes called the waveguide self-imaging period(WSIP) and sometimes symbolized as D_(w) for distance between successivefocal planes or waists, is related to the index of refraction (n) of thewaveguide propagation medium, the width or thickness (a) of thewaveguide propagation medium, and the wavelength (λ) of the light beingpropagated. In general, WSIP=4 na²/λ, although in the special case wherethe beam is symmetric with respect to the center of the waveguide, i.e.,the beam profile is identical with its mirror image, WSIP=na²/λ). See,e.g., Bryngdahl, J. Opt. Soc. Am 63, 416 (1973); Ulrich, R. and Aukele,G., Appl. Phys. Lett., 27, 337 (1975).

This invention utilizes that re-imaging principle of rectangular andother parallelepiped waveguides to construct the high-power opticamplifier 10 in a manner that can deliver a high-power, diffractionlimited, output beam 14 with a desired spatial coherence and amplitudeprofile, preferably lower order super-Gaussian, as described above. Theinput beam 12 with the desired spatial coherence and amplitude profileis injected at an entrance face 26 into the rectangular, multi-mode,waveguide 20, which has a core 22 comprising an optical gain medium or a3- or 4-wave mixing, nonlinear medium, i.e., a material that can beexcited or populated with additional energy that then gets added to thelight energy in a beam that passes through the material (hereinaftersometimes referred to generally and interchangeably as a “gain medium”or “mixing medium”). Therefore, the input beam 12, upon entering therectangular, multi-mode, waveguide core 22, breaks into as manyeigenmodes as the width or thickness (a) and numerical aperture of thewaveguide 20 will allow for the light wavelength, while it getsamplified with the additional energy. Input of the additional energy tothe core 22 can be accomplished with any of a number of well-knowntechniques, which are symbolized generically by the energy input arrows30, 32 in FIG. 4. Therefore, while the multi-mode propagation of thelight beam 12 through the waveguide 20 degrades in beam quality asvarious modes of propagation mix and interfere in the waveguide 20, thelight beam 12 never-the-less gets amplified with additional energyacquired from the excitation or pump energy 30, 32 in the gain medium ofthe core 22. However, according to the re-imaging characteristic ofrectangular waveguides described above, the input spatial profile of thebeam 12 is reproduced periodically at specific distances of propagation,i.e., WSIP=4 na²/λ (or WISP=na²/λ in the symmetric case), and suchreproduction of the desired spatial profile occurs even though the beamis amplified with additional energy acquired in the gain medium of thecore 22. Therefore, the exit plane or aperture 28 of the multi-mode,rectangular waveguide core 22 is positioned, according to thisinvention, to coincide with a re-imaging plane of the waveguide 20,i.e., at some positive, non-zero integer (i) multiple of the waveguideself-imaging period (WSIP). Consequently, the length of core 22 of thewaveguide 20 extending between the entrance face 26 and the exit face 28is equal to WSIP×i, where i a positive, non-zero integer, such as 1, 2,3, . . . , etc. The result is an amplified output beam 14 at the exitface 28 with the same spatial profile as the input beam 12 at theentrance face 26. Effective thermal management and other power scalingfeatures and capabilities of such devices, according to this invention,will be discussed in more detail below.

Since the cross-sectional area of the core 22 of a multi-mode waveguide20 can be so much (orders of magnitude) larger than the cross-sectionalarea of a single-mode waveguide (not shown) the energy inputs 30, 32 andresulting beam amplification in the optical amplifier 10 can be muchgreater than would be possible in a typical single-mode waveguideoptical amplifier. Yet, the highly amplified output beam 14 of thehigh-power optical amplifier 10 of this invention can retain the desiredinput beam 12 wavefront and spatial intensity profile, for example, aspatially coherent, lower order super-Gaussian profile.

Further, the ability to use a much larger cross-sectional area for thecore 22 of the multi-mode, rectangular optical amplifier 10 of thisinvention accommodates much higher power amplification capabilitieswithout the adverse non-linear effects of, for example, stimulatedBrillouin scattering, stimulated Raman scattering, thermally-inducedphotoelastic effects, self-phase modulation, and four wave-mixing, whichare typically encountered in single-mode waveguide amplifiers. Theability to provide superior thermal management with rectangular,multi-mode waveguides 20, as will be discussed in more detail below,also mitigates stress birefringence, which otherwise degradespolarization of the propagating light beam, and self-phase modulationdegrades temporal and spatial coherency of the beam. For example,polarization maintenance in large effective core and double clad filteramplifiers is intrinsically difficult due to the circular geometry andradial thermal profiles. Therefore, the ability to provide high-poweroptical amplification with the amplifier 10 of this invention, whilestrongly mitigating the adverse effects of stress birefringence andnonlinear optical distortion, enables the amplifier 10 to produce a highintensity output beam 26 that not only retains temporal and spatialcoherence of the input beam 12, but that also retains any polarizationof the input beam 12, which is important for many signal processing,beam steering, industrial cutting, optical phased arrays, and otherapplications of high intensity laser beams.

The beam source 16, input lens 18, and output lens 19 in FIG. 4 areshown only to illustrate, in a symbolic manner, a typical opticalinput/output arrangement for an optical amplifier and are not intendedto be limiting in any way, since persons skilled in the art are wellaware of, and capable of providing, myriad such input/output systems andconfigurations. Suffice it to say that operation of the amplifier 10requires an input beam 12 from some source 16, which could be a laserdiode or any other light beam producing apparatus or simply a light beambeing transmitted by an associated system, such as a free-space system,optical fiber, or other waveguide propagated beam that is to be coupledinto, and amplified by, the amplifier 10 of this invention. In the caseof an amplifier that is outfitted with resonator optics to use theamplifier as a laser resonator, the input beam may comprise the portionof the light that is reflected by the resonator optics back into theamplifier, as will be described in more detail below, for example, inrelation to FIG. 11. The input lens 18 in FIG. 4 is symbolic of anyoptical components or system that shapes, conditions, and/or focuses theinput beam 12 with the desired spatial coherence and amplitude profileon the aperture or entrance face 26 of the core 22. An output lens 19 isnot really required for an amplifier 10, but is shown here only assymbolic of any myriad optical components or systems that may be used tocouple the amplified output beam 14 to an optic transport system, suchas the connecting waveguide 322 in FIG. 1, or other devices or toproject the amplified output beam 14 into free space.

The large cross-sectional area of the core 22 also accommodates a widevariety of energy input or pumping systems, which are indicatedsymbolically in FIG. 4 by arrows 30, 32, and from any side or end of thecore 22, although side pumping, as indicated by arrow 30 is particularlyadvantageous and preferred according to this invention, as will bediscussed in more detail below. The gain medium of core 22 can even beoptically pumped by poor beam quality optical sources, such as abroad-stripe laser diode emitter or array coupled into a lateral side ofthe waveguide 20, as indicated by arrow 30, since the pump beam does nothave to be focused into the entrance aperture or face 26, as would berequired for a single-mode waveguide.

Optical confinement in the rectangular, multi-mode, waveguide 20 can beprovided by any of myriad well-known waveguiding techniques. Forexample, total internal reflection by a cladding 24 with a lower indexof refraction than the core 22 is one typical waveguiding technique, asis reflective surfaces on waveguide walls (not shown in FIG. 4). Aparticularly advantageous non-clad core 22 embodiment will also bedescribed in more detail below.

Evanescent field leakage or propagation effects also have to beconsidered in determining the distance (WSIP×i) between input and outputapertures or end faces 26, 28 of dielectric core, self-imagingwaveguides 20. As illustrated in FIG. 8, light in various propagationmodes, for example modes 370, 372, propagating through the waveguide 20generally in the direction of the longitudinal axis Z are turned byTalbot refraction at the interfaces 374, 376 of the core 22 with thecladding 24 when the index of refraction of the core 22 is less than theindex of refraction of the cladding 24. However, such Talbot refractionis not the same as perfect reflection and does not occur completely at aparticular point for any particular ray 370, 372. Instead, there is someevanescent leakage 371, 373 into the cladding 24, which can cause there-phasing distance along the longitudinal axis Z, i.e., WSIP, to beslightly shorter than if there was total reflection at discrete pointsat the interfaces 374, 376. Such change in re-phasing distance is due toincrease of length of travel for the light in the evanescent field ofthe cladding 24, as indicated at 371, 373. The extent of such re-phasingdistance change due to evanescent effects will vary depending on thecore 22 and cladding 24 materials and their respective indices ofrefraction, wavelength of the light, and dimensions of the waveguide 20.

Opposing waveguide walls do not have to be strictly parallel, althoughso-called tapered waveguides would have continuously variable imagingperiods. Therefore, there would be more than one WSIP for a taperedwaveguide. Consequently, placement of an exit face 28 of a taperedwaveguide at some integer multiple of one WSIP, as described above forparallel waveguide walls, might miss positioning the exit face 28 at are-image plane, thus would not be appropriate or effective. However, byknowing any particular degree of taper, the location of a re-image planecan be determined, and the exit face 28 can be positioned in such are-image plane according to this invention. Therefore, use of a taperedwaveguide with a rectangular cross-section for a beam amplifier andpositioning the exit face at any re-image plane is considered to beequivalent and part of this invention, even if the exit face positionmight not be at a simple integer multiple of a WSIP in a taperedwaveguide embodiment. A tapered, self-imaging waveguide can be useful inpower scaling and coupling dissimilar self-imaging waveguides.

The waveguide core 22 can comprise any gas, liquid, or solid gainmedium, although a solid gain medium is preferred for high energydensity and thermal conductivity, which are essential for scaling up tohigh powers in laser amplifiers and resonators. However, gas thermalconductivity is about 10⁻² to 10⁻³ less than thermal conductivity ofsolids. Therefore, as gain media, such as the system disclosed anddescribed in U.S. Pat. No. 5,684,820, issued to Jenkins et al., areill-suited for such purposes according to this invention, because theycannot dissipate heat efficiently enough to handle the high power levelsfor which this invention is designed and, further, because thedimensions are not generally amenable to high gas flow rates that couldenhance heat exchange. Essentially, a gain or mixing medium is amaterial that can impart energy to a light beam that propagates throughit. Since many such gain or mixing media are well-known to personsskilled in the art, it is not necessary to further explain or describethem for an understanding of this invention. Several examples willsuffice, such as Nd:YAG (yttrium aluminum garnet doped with neodymium),Nd:YLF, or homologous materials, AlGaAs, In GaAsP, or varioussemiconductor materials, Nd-doped phosphate glasses, or CS₂, which is awell-known nonlinear optical response liquid that is often used forsuper-continuum generation.

The quasi-one-dimensional, rectangular waveguide 20 geometry illustratedin FIGS. 4 and 5 is particularly attractive for high-power waveguideapplications, since the aperture 26, 28 and shape of the core 22 isconducive to the desired lower order super-Gaussian amplitude profile350 (FIG. 2) described above, and since the aperture area of theentrance face 26 can be over ten thousand (10,000) times larger thanthat of a typical single-mode optical fiber, which allows for high-poweroperation of the amplifier 10 of this invention at intensities andfluences that are well below the threshold for optical damage andnonlinear optical effects in the waveguide 20, while still maintainingpolarization and beam spatial coherence and amplitude profile, asdiscussed above. The potential for peak power scaling, i.e., scaling upto high-power operation, is proportional to the waveguide aperture areaat the core face 26, which, as mentioned above, is not restricted inthis rectangular, multi-mode, waveguide 20 by typical single-modeaperture constraints. Additionally, the ability to scale average powerto high levels is dependent to a large extent on the thermalconductivity and surface area of the core 22 and cladding 24, and theflat-sided, rectangular shape of these rectangular, self-imaging,waveguides are advantageous for thermal management and heat dissipationaccording to this invention. The quasi-one-dimensional waveguide 20geometry of FIGS. 4 and 5 is particularly attractive for high-powerwaveguide amplifier applications, since this geometry has: (i) an end orside aperture that can be coupled to one or more laser diode arraysallowing optical pumping with high coupling efficiency, which will bedescribed in more detail below in relation to FIG. 8; (ii) a highsurface-to-volume ratio for efficient heat dissipation, as will also bedescribed in more detail below in relation to FIG. 13; and (iii) lowthermal gradients that are essentially one-dimensional due to thinwaveguide 20 geometry. For example, a 1-cm long, one-dimensional, laserdiode array for high coupling efficiency of the laser diode output tothe waveguide, as defined by getting the majority of the diode pumplight, preferably waveguide 95% or more, into the core. The ability touse materials, such as sapphire and yttrium aluminum garnet (YAG) in therectangular, self-imaging waveguides of this invention, as opposed tothe silicate, germinate, or phosphate glasses used in fibers, alsoenhances the thermal management and heat dissipating abilities for powerscaling according to this invention, as will be described in more detailbelow.

A quasi-one-dimensional waveguide 20 geometry, such as that illustratedin FIG. 5, sometimes simply called “one-dimensional” for short, isgenerally considered to be one in which there is self-imagingwaveguiding in the direction of only one transverse axis 21 and simplyfree-space propagation along the unguided direction or transverse axis23. In other words, the waveguide width or thickness (a) along onetransverse axis 21 (the distance between core/cladding interfaces 25,27) is much smaller than the width along the other transverse directionor axis 23 (the distance between core/cladding interfaces 35, 37). Whenthere is enough difference between the direction 21 width or thicknessand the direction 23 width or thickness so that the beam width in thecore 22 is always smaller than the 23 direction width, and when the beamdivergence along the short axis 21 is large enough compared to thewaveguide width in the direction of axis 21 that the beam spreads andreflects from interfaces 25, 27 causing imaging along axis 21, thewaveguide 20 is considered to be quasi-one-dimensional. In other words,if the length of the waveguide is such that self-imaging obtains in onedirection and substantially free space propagation (or very longre-phasing distances) describes the other direction, thenquasi-one-dimensional waveguides are appropriate. As such, aone-dimensional waveguide can also have a core 22 sandwiched between twocladding pieces, such as the top piece 24 a and 24 b of FIG. 5, withoutthe wrap-around, edge cladding pieces 24 c and 24 d.

A two-dimensional, rectangular waveguide 20 cross-section is shown inFIG. 6. The core 22′ is surrounded by a reflective cladding 24′ toconfine the light beam to the core region 22′. The cladding 24′ can be,for example, a medium of lower index of refraction than the core region22′, for achieving total internal reflection. It could also be a mediumof intrinsic reflectivity at the beam propagation wavelength, such asmetal, air, vacuum, or one or more dielectric coatings that reflect thepropagation wavelength. Reflections along one transverse direction oraxis 21′ occur at the opposite core/cladding interfaces 25′, 27′, whilereflections along the other transverse direction or axis 23′ occur atthe opposing waveguide interfaces 35′, 37′. The waveguide self-imagingperiod (WSIP) from each set of opposing waveguide interfaces 25′, 27′and 35′, 37′ will be different, if the rectangular core 22 cross-sectionis not a square. On most applications, it is desirable that there-phasing or re-imaging distances (WSIP×i) for the two waveguide axes21′, 23′ match at the waveguide aperture or faces 26, 28. However it maybe useful in certain situations to have one of the waveguide axes be ata “fractional Talbot distance” in order to split the exit beam intomultiple, power-divided replicas of the input beam.

An illustration of a square cross-section waveguide 20″ is shown in FIG.7. In this embodiment, the waveguide width along its vertical axis 21″,i.e., the distance between core/cladding interfaces 25″, 27″, and thewidth along horizontal axis 23″, i.e., distance between core/claddinginterfaces 35″, 37″, are equal. Therefore, the waveguide self-imagingperiod (WSIP) relative to each of these axis 21″, 23″ is the same.

References herein to rectangular waveguides and rectangular aperturesalso apply to square cross-sections and square apertures as well as toone-dimensional or quasi-one-dimensional waveguides and apertures,unless otherwise noted, and any of which may be rigid structures, slabs,ribbons, thin films, hexagons, parallelepipeds, and other self-imagingstructures, such as polygons with parallel opposing sides of equaldimensions, but to all sides necessarily being the same length. Also,apertures refer to the end faces, e.g., entrance and exit faces 26, 28(FIG. 4) of the core 22, which are essentially the surfaces or optical“openings”, where light beams 12, 14 enter and exit the core 22. Inother embodiments of this invention described below, apertures or launchapertures may also refer to openings or devices in lateral sides, tops,or bottoms of waveguides as well as end faces or openings where light iscoupled, propagated, or launched into or out of the waveguide.

As explained above, spatially coherent, lower order super-Gaussian,beams are particularly desirable for the power scalable, rectangular,self-imaging, waveguide optical systems of this invention, because theycan have more light energy than common TEM₀₀ Gaussian beams of similarsize, and they can be adapted to utilize more fully the rectangularcross-section of rectangular waveguides, especially one-dimensionalrectangular waveguides. For example, waveguides with thicknesses in therange of 100 μm to 1 cm or more combined with a super-Gaussian beam canbe scaled up to produce and/or transport the high power levels discussedabove according to this invention. A preferred example very high peakpower self-imaging waveguide according to this invention is a 1 cm×1 cmcore with a lower order super-Gaussian beam. A spatially coherent, lowerorder super-Gaussian beam 12 for focusing on a rectangular inputaperture 26, such as that shown in FIG. 4, can be provided by amplitudeor phase modification of another beam format being injected into anoptical amplifier or laser resonator via binary optics, as describedabove, by an apodized plane wave injection source, by a super-Gaussianmode resonator master oscillator, or by other methods or devices knownto persons skilled in the art. A phase modification plate 351 isillustrated in the input beam 42 of the amplifier 40 in FIG. 9 and theamplifier 40′ in FIG. 10 along with an amplitude modification plate351′. Either one, both, or more of such plates 351, 351′ can be used tomodify the beam amplitude format or profile of input beam 42 to asuitable super-Gaussian amplitude profile for amplified output beam 44,as explained above and as would be understood by persons skilled in theart. Such a phase modification plate 351 is also shown positioned inbeam 72 of the laser resonator 70 in FIG. 11 to produce a super-Gaussianoutput laser beam 74. Again, an amplitude modification plate could beused in beam 72, and such an amplitude and/or phase modification platecould also be positioned in beam 72′ in addition to, or instead of, thephase modification plate 351 or amplitude modification plate (not shown)in beam 72. While such binary optics as phase modification plate 351and/or amplitude modification plate 351′ are illustrated only in theamplifier 40 in FIG. 9, in the amplifier 40′ embodiment of FIG. 10 andin the resonator embodiment 70 of FIG. 11, they or any other appropriatebeam modifying devices can be used in conjunction with any other laseramplifier or resonator configuration described herein or otherwise knownin the art to produce super-Gaussian beams for purposes described inrelation to this invention. Also, appropriate positions of phasemodification plate 351 and/or amplitude modification plate 351′ are notlimited to those shown or described in FIGS. 9, 10, and 11, since thebeam can be modified to super-Gaussian in many locations in relation toother optical components associated with laser amplifier and resonatordevices either before or after passing the beam through the core or gainmedium.

Of course, as mentioned above, additional considerations and designswith appropriate materials, thermal management and heat extraction ordissipation are also important for average power scaling according tothis invention. For example, a one-dimensional parabolic thermal lenswill form in the self-imaging waveguide core, if the core is too large.If the thermal lens focal length becomes shorter than, or commensuratewith, the re-phasing distance, then the self-imaging propagation can becorrupted. Therefore, the most advantageous self-imaging waveguidedimensions for average power scaling are a complex combination of gainmedia, thermal-optic properties, pump conditions, and heat extractiondesign. For example, Nd:YAG and Yb:YAG waveguides typically would havedifferent absorption and stimulated emission cross sections, as would beunderstood and well within the capabilities of persons skilled in theart.

The high-power optical amplifier 10 of this invention shown in FIG. 4and described above is one simple embodiment illustrating the use of amulti-mode, rectangular, re-imaging waveguide with a core comprising again or mixing medium for amplifying a light beam 12 or mixing withother beams. There are, of course, many other useful amplifierconfigurations and embodiments that will occur to persons skilled in theart, once they understand the principles of this invention. For example,as shown in FIG. 9, a double-pass optical amplifier embodiment 40, inwhich the light beam 42 being amplified is passed twice through therectangular, multi-mode, self-imaging, waveguide 50 to thereby extracteven more energy from the gain medium in the waveguide core 52. In thisexample, the gain medium that comprises the core 22, such as Nd:YAG, ispumped with light energy 64 with, for example, a laser diode 60 coupledinto the core 22 by a lens focusing system 59, which focuses the light64 produced by the laser diode 60 onto the end face of aperture 56 ofthe core 52. The end face 56 is coated or covered with a dichroicmaterial or combination of materials 66 that is highly transmissive ofthe wavelength of light 64 produced by the laser diode 60, but highlyreflective of the light 42 that is to be amplified. Such dichroicfilters 66 are well-known in the art and will be selected based on thewavelengths of the pump light 64 and the input beam 42.

The input beam 42 follows the same path 43 into and out of the waveguide50, so the amplified output beam 44 has to be separated from the inputbeam 42. A common, well-known system for combining an input light beam42 and output beam 44 of the same optical path 43 and then separatingthem includes a polarizing beam splitter 45, which comprises abirefringent prism or other layer of material 46, which reflects lightthat is polarized in one plane, e.g., the s-polarization plane, andtransmits light that is polarized in the orthogonal plane, e.g., thep-polarization plane. Thus, the input beam 42, in this example, may bes-polarized so that the polarization selective layer 46 in the beamsplitter 45, reflects the input beam 42 into the optic path 43, which isaligned with the face or aperture 58 of the waveguide core 52. Abirefringent, ¼-wave retarder 47 is positioned in the beam path 43, sothat the reflected input beam 42 has to pass through the ¼-wave retarder47 on its way to the waveguide 50. The ¼-wave retarder 47 converts thes-polarization of the input beam 42 to circular polarization. The lenssystem 49 focuses the input beam 42 onto the aperture or face 58 of thecore 52, again, with whatever spatial profile is desired. The lenssystem 49 is symbolic of any desired lens system that could be used forany desired beam profile, including, but not limited to, the desirablespatially coherent, lower order super-Gaussian amplitude profiledescribed above.

As the input beam 42 propagates through the gain medium of the core 52of the multi-mode waveguide 50, it breaks into as many modes ofpropagation as the size and numerical aperture of the rectangular core52 permits for the wavelength of the light in the beam 42, and itacquires or extracts energy from the gain medium in the core 52 that issupplied by the pump light 64, thereby being amplified. At the sametime, as described above, the rectangular waveguide periodicallyreconstructs or re-images the input beam spatial profile that is focusedby the lens system 49 onto the aperture or face 58 at non-zero, positiveinteger (i) multiples of the waveguide self-imaging period (WSIP), i.e.,at WSIP×i. Therefore, as described above, the apertures or faces 56, 58of the core 52 are positioned at locations that coincide with re-imageplanes and spaced apart from each other by a distance equal to WSIP×i.The reflecting surface 56 could also be at a fraction of a WSIP distanceas long as the total distance between faces 56, 58 and back again isWSIP×i. Upon reaching the end face 56, the amplified input beam 42 isreflected by the dichroic coating material 66 for another pass backthrough the core 52, where it continues to acquire or extract even moreenergy from the gain medium of the core 52. Therefore, the beam isamplified again on this second pass through the core 52. Again, withproper spacing of the faces 56, 58, the twice amplified beam isre-imaged at face or aperture 58 with the same spatial profile as it hadwhen it entered aperture or face 58. Therefore, the amplified outputbeam 44 has a gain modified spatial profile, for example, a spatiallycoherent, lower order super-Gaussian beam, as the input beam 42,according to this invention. In some cases, a regular Gaussian beaminput can emerge as super-Gaussian due to gain saturation, which is alsowithin the scope of this invention.

Again, the amplified output beam 44 emanates from the waveguide 50 alongthe same optical path 43 as the input beam 42, so it has to be separatedfrom the input beam 42 in order to direct it to whatever application(not shown in FIG. 5) for which it is amplified. Such separation isaccomplished by the ¼-wave retarder 47 and polarizing beam splitter 46.Since one of the attributes of this invention is that the light beam 43maintains its polarization as it gets amplified in the multi-mode,rectangular waveguide 50, it emerges from the aperture or face 58 withthe same circular polarization that was imparted to the input beam 42 bythe ¼-wave retarder 47, as described above. Upon re-passing back throughthe ¼-wave retarder 47, the polarization of the light beam is rotatedagain to polarization in the p-polarization plane, which is orthogonalto the s-polarization plane of the input beam 42. Therefore, the nowp-polarized output beam 44 is transmitted, instead of reflected, by thepolarization selective layer 46 in polarizing beam splitter 45, and itemerges from the polarization beam splitter 45 on a different outputpath than the path of the input beam 42. Of course, persons skilled inthe art will recognize that an input beam with p-polarization and theoutput beam with s-polarization can also be used to the same effect, andthere are other suitable optic arrangements for feeding input beams intoand extracting output beams from state-of-the-art optic amplifiers andother optic components that would work with the rectangular, multi-mode,self-imaging, waveguide amplifier 40 of this invention. Moreover,differences or variations in WSIP distances due to differences inevanescent field penetration in the cladding usually only results insmall residual wavefront astigmatism and curvatures that can becorrected or removed easily with mode matching optics, as would beunderstood by persons skilled in the art.

The double-pass amplifier embodiment 40′ shown in FIG. 10 is similar tothe FIG. 9 amplifier embodiment 40 described above, except that acurved, dichroic mirror 66′ is used instead of the dichroic coating 66in the FIG. 9 embodiment to admit pump light 64 into the gain medium ofcore 52 and to reflect the once-amplified input beam 42′ back into thewaveguide 50 for a second amplification pass through the gain medium ofcore 52. The curved, dichroic mirror 66′ is spaced a distance away fromthe aperture or face 56 and is curved to re-focus the once-amplifiedbeam 42′ emerging from face 56 back into the aperture or face 56 withthe desired spatial profile, allowing optical devices, such aselectro-optic switches (not shown) and saturable absorbers (not shown)to be inserted. As would be understood by persons skilled in the art,the dichroic mirror 66′ in FIG. 10 or the dichroic coating 66 in FIG. 9could be constructed to perform some or all of the beam modificationsnecessary to produce a super-Gaussian output beam 44 instead of, or inaddition to, the phase modification plate 351 and/or the amplitudemodification plate 351′, if desired. The same can be said for otheroptical components, such as the lens 49, retarder 47, or beam splitter45.

As is well-known in the art, a laser resonator is quite similar to, andcan be considered a special category or application of, an opticalamplifier. To use an optic amplifier as a laser resonator, reflectorsare used at opposite ends of an amplifier gain medium to reflectamplified light back and forth through the amplifier gain medium foradditional re-amplification. Therefore, a rectangular, multi-mode,self-imaging, waveguide can also be used according to this invention toprovide power scalable, multi-mode, laser resonators that produce outputbeams with desired spatial profiles, e.g., with lower ordersuper-Gaussian or common TEM₀₀ Gaussian output beam profiles.

An example laser resonator 70, which includes a multi-mode, rectangular,self-imaging waveguide 80 amplifier according to this invention, isshown in FIG. 11. Essentially, a multi-mode, rectangular, self-imagingwaveguide 80 with a core 82 comprising an optical gain medium, such asNd:YAG or any other suitable gain medium material, is excited or pumpedto emit light energy 72, 72′ from opposite faces or apertures 86, 88 ofthe core 82. The gain medium of core 82 can be excited or pumpedoptically with light energy from a laser diode 90, as shown in FIG. 11,or from any of myriad other known light energy sources, or, if the gainmedium is a laser diode compatible material, it can be excitedelectrically to emit light energy 72, 72′. A laser diode compatiblematerial and structure is one in which electrically pumped gain mediumproduces optical gain and thereby coherent light as an output. Personsskilled in the art are aware of, and capable of implementing, suchexcitation or pumping techniques, so it is not necessary to explain themhere for an understanding of this invention. In the laser resonator 70illustrated diagrammatically in FIG. 11, the pump light 94 from laserdiode 90 is focused into the aperture or face 86 of the core 82 by alens system 92, although it could be coupled into the core 82 throughany other side surface, as explained above in relation to the opticalamplifier illustrated in FIG. 4.

The curved mirror or reflector 96 is dichroic in this FIG. 11arrangement, similar to the reflector 66′ in FIG. 10, to transmit pumplight 94 of a wavelength produced by the laser diode 90, but to reflectlight energy 72 of a wavelength emitted by the core 82 of waveguide 80.Therefore, light energy 72 emitted from the aperture or face 86 of core82 is reflected by the mirror 96 back into the core 82.

The curved mirror or reflector 79 adjacent the opposite face or aperture88 is partially reflective of light energy 72′ of a wavelength emittedby the gain medium of core 82. For example, but not for limitation, themirror 79 may be fabricated to reflect about 80 to 90 percent of thelight energy 72′ and to transmit about 10 to 20 percent of such lightenergy as output beam 74. Thus, about 80 to 90 percent of the lightenergy 72, 72′ emitted by the core 82 will be reflected or resonatedback and forth through the gain medium of core 82 many times, and witheach such pass through the gain medium of core 82, the light energy 72,72′ is further amplified with energy derived by the pump light 94.Therefore, the light beams 72, 72′ become very intense, and, the outputbeam 74 also becomes very intense. According to this invention, thewaveguide 80 is multi-mode, so the core 82 can be very large, whichenables it to handle high energy levels and still maintain its opticaland structural integrity. Either the totally reflected beam 72 or theportion of the beam 72′, or both, can be considered an input beam to theamplifier core 82 and either or both of such reflected or input beams72, 72′ can be conditioned or modified to produce a super-Gaussian, acommon TEM₀₀ Gaussian, or any other desired beam profile.

Also, according to this invention, the curved mirrors and/or phaseplates 96, 79 are shaped and spaced from the respective core aperturesor faces 86, 88 in such a manner as to condition and focus the lightenergy 72, 72′ onto the faces 86, 88 with a desired spatial profile, forexample, a preferred, spatially coherent, lower order super-Gaussianbeam profile or a common TEM₀₀ Gaussian profile. For a preferred,spatially coherent, lower order super-Gaussian beam, an optical systemsimilar to that described above for amplifiers can be used for this andother laser resonators. Also, either of the mirrors or phase plates 96,79 can be constructed to perform the phase and/or amplitudemodifications necessary to produce a desired super-Gaussian profileinstead of, or in addition to, the phase modification plate 351.Further, according to this invention, the core 82 length extendingbetween opposite faces 56, 58 is a non-zero, positive integer multipleof the waveguide self-imaging period (WSIP), i.e., a length equal toWSIP×i. Therefore, even though the light energy 72, 72′ reflected backinto the waveguide 82 breaks into as many modes of propagation as thedimensions and numerical aperture of the rectangular waveguide 80 willallow for the wavelength of light being produced, thus interferes andchanges spatial profile as it is being amplified in the gain medium ofthe core 82, it always re-images for emission at the apertures or faces86, 88 with the same desired spatial profile as was shaped for input bythe mirrors 96, 79. Therefore, the intense, highly amplified, outputlaser beam 74 will have spatial as well as temporal coherence with adesired spatial profile, such as the preferred super-Gaussian or thecommon TEM₀₀ Gaussian beam.

While it is not shown, the dichroic mirror 96 in FIG. 11 could bereplaced by a dichroic coating, such as the dichroic coating 66 shown inthe amplifier in FIG. 9, on aperture or face 86, or the partiallyreflective mirror 79 could be replaced by a partially reflective coating(not shown) on the aperture or face 88. However, something on oradjacent at least one of the ends or faces 86, 88 has to focus orotherwise provide the desired beam spatial profile on at least one ofthe faces or aperture 86, 88. Therefore, at least one of the curvedmirrors 96, 79 is needed to provide free-space focusing of a beam 72,72′ on a core face or aperture 86, 88, or some other optic arrangementto provide the desired spatial profile on an aperture or face 86, 88would have to be provided. For example, one or both of the reflectivecoatings discussed above could be graded reflectively across its surfaceto reflect only a super-Gaussian spatial profile back into the core 82.

An alternate embodiment optical amplifier shown diagrammatically in FIG.12 illustrates use of a rectangular, multi-mode, self-imaging, waveguide20′ in an amplifier 10′ that is similar to the amplifier 10 in FIG. 4,but with a non-clad core 22, which can also be used with any of theinput/output optics and laser resonator equipment and processeddescribed above. For example, a core 22 comprising a gain medium ofNd-doped phosphate glass has an index of refraction that is high enoughin relation to air or a gas (1.0), such as argon or nitrogen, atatmospheric pressure to provide total confinement of a light beam 12 inthe core 22 positioned in such an atmosphere without any other solid orliquid cladding or reflective material on the surfaces of the core 22.Other core materials that are transparent to the light beam 12 and havea high enough index of refraction to provide such total confinement in agas atmosphere could also be used in this manner. Any of the amplifieror laser resonators disclosed in FIGS. 1–11 and described above can beimplemented with the non-clad core 22 shown in FIG. 12. Such amplifiersand laser resonators are particularly advantageous for heat dissipation,because the core is not covered by cladding, which can inhibit heat flowout of the core 22, where the heat is created from absorbed lightenergy. However, mechanical robustness and fabrication considerationsfavor clad self-imaging waveguides, especially in larger power scaledconfigurations.

For the rectangular waveguides 20, 50, 80 of the amplifiers and laserresonators described above and shown in FIGS. 1, 5, 6, and 7, tofunction effectively and reliably at high power levels, waveguidematerials are important. A dopable, solid, gain material for the core22, 52, 82 can be either a doped dielectric or doped semiconductor. Adielectric material is generally considered to have a bandgap of morethan 3 eV at room temperature, and common semiconductor materials aregenerally considered to have a bandgap in the range of 0.25 to 1.1 eV,although many semiconductor materials are also available with bandgapshigher than 1.1 eV and reaching as much as 3.0 eV. For clad waveguides20, 50, 80, the core material 22, 52, 82 is preferably combined with acladding material 24, 54, 84 that not only has a lower index ofrefraction for reflectivity and wave guidance as explained above, butwhich also has a comparable coefficient of the thermal expansion,comparable thermal conductivity, high tensile strength, and capabilityof bonding to the core material in order to withstand thermal effectswhile dissipating heat. In general, oxides, such as YAG (yttriumaluminum oxide), are bondable to other oxides, such as sapphire (Al₂O₃),and chalcogenides are bondable to other chalcogenides. However,fluorides are not generally bondable to oxides.

The thermal conductivity of both the solid core material 22, 52, 82 andthe cladding material 24, 54, 84 should be high, e.g., about 1watt/cm·K, which is orders of magnitude more conductive than a gas core.However, in order to dissipate heat produced in the core 22, 52, 82efficiently and to prevent hot spots of local heat concentrations, it isalso important that the conductivity of the core and cladding match ornearly match each other. As a general guide, heat conductivitydifference between the core material 22, 52, 82 and the claddingmaterial 24, 54, 84 should be no more than about 25 percent of thecladding material conductivity in order to avoid thermally-induced,interfacial stresses that cause optical distortions and ultimately, canlead to material failure.

As the power increases, there is an ever increasing need for a match ornear match between coefficient of thermal expansion of the core material22, 52, 82 and coefficient of thermal expansion of the cladding material24, 54, 84 in order to avoid excessive stress and resulting separationof the cladding material from the core material and/or breakage ofeither the core material or the cladding material during heating andcooling. Again, as the power increases, any difference in coefficientsof thermal expansion between the core material and the cladding shouldbe a low percent, e.g., 20 percent or less, of the coefficient ofthermal expansion of the core material. Core and cladding materials withhigh tensile strength, such as at least 100 megapascals, e.g., YAG, alsoreduces likelihood of fracture or breakage during heating and cooling.If the core material 22, 52, 82 comprises a semiconductor material thatis pumped electrically to produce pump light energy, the claddingmaterial 24, 54, 84 may also have to be an electrically conductivematerial to serve as an electric contact.

In a preferred embodiment, the core material 22, 52, 82 comprises aneodymium-doped yttrium aluminum garnet (Nd:YAG) dielectric materialclad by aluminum oxide (Al₂O₃), also known as sapphire. The Nd:YAG,often a bulk slab about 100 μm thick, has a coefficient of thermalexpansion of about 7.5×10⁻⁶/° C. and tensile strength in the range of125–200 megapascals. Sapphire has a coefficient of thermal expansion inthe A crystal lattice direction of 6.65×10⁻⁶/° C. and 7.15×10⁻⁶/° C. inthe C crystal lattice direction. Therefore, for purposes of thispreferred embodiment, the cladding material slabs are each cut from bulksapphire along the C lattice direction and is laminated onto the Nd:YAGcore material with the sapphire C direction oriented in the length Ldimension of the core to minimize thermal stress. Sapphire has a tensilestrength of about 300 megapascals.

As mentioned above, good thermal management of optical systems, such aslaser amplifiers, resonators, and transport waveguides, is essential forscaling to high optical power levels of operation, and the rectangular,multi-mode, self-imaging, waveguide amplifiers and laser resonators 10,40, 50 70, and 10′ are especially adaptable to excellent heatdissipation and thermal control according to this invention. Heat isgenerated primarily by energy absorbed from light in the cores 22, 52,82, of those amplifiers and laser resonators. The more light energy thatis pumped into, or created in, such cores, the more heat will begenerated. If such heat is not dissipated, but allowed to build in thecore to unacceptable levels, performance is adversely affected, first,because of optical distortions due to thermal gradients in the corematerial, and, ultimately, catastrophic (structural) failure due tomelting and/or vaporization of the core material, especially at theapertures or other focal planes where light energy is most concentrated.

As already mentioned above, solid core materials, such as sapphire, havethermal conductivities that are orders of magnitude higher than gaseouscore materials, thus are much more conducive to scaling up to high powerlevels. Also, as mentioned above, a claddingless core 22, as illustratedin FIG. 12 may be better able to dissipate heat, because heat transferis not inhibited by a surrounding cladding, although such inhibition canbe mitigated in clad structures by providing cladding that has thermalconductivity at least as great the thermal conductivity of the core.

However, in addition to the advantages of solid core materials for heatdissipation, the flat-sided shapes and dimensions of rectangular,multi-mode, self-imaging, waveguide cores are very well-adapted to heatdissipation according to this invention. For example, as illustrated inFIG. 13, the large surface area of the waveguide (or cladding) walls130, 131, which may or may not be optically transparent to the pump andlaser wavelengths, facilitate coupling heat sinks 112, 112′ for thermalmanagement of the amplifier 100 by enhancing removal of heat from thecore 22. Therefore, the term heat sink, as used herein, means,generally, any device by means of which heat is absorbed in or removedfrom the core 22 (and cladding in clad amplifier embodiments). It can bea traditional heat sink in the sense of a body or environment having agreater heat capacity and a lower temperature than the core (cladding,if the core is clad) with which it is in contact. It can also includeheat spreaders (e.g., diamond), optically transparent cladding, oropaque, actively cooled heat sinks. The spreader, cladding, or heat sinkcan be used alone or in combination to achieve thermal management of thecore 22. Moreover, the thickness of the spreader, cladding, and/or heatsink can be varied across the waveguide surface to produce a moreuniform temperature profile across the waveguide. For example, thebottom surface 113 of the heat sink 112 can be tapered, as indicated at113′ in FIG. 13, to extract more heat from the portion of the core 22that is near the thicker end of a tapered heat sink 112 than from theportion of the core 22 that is near the thinner end of a tapered heatsink 112. If the waveguide amplifier 100 produces more heat adjacent thethicker end, such as due to more pump light 122 energy or some otherheat build-up cause, the higher heat extraction capability of thethicker end of the heat sink 112 can help to maintain a more uniformtemperature profile in the length of the waveguide core 22. Other shapesthan the taper can be used, such as curved bottom surface 113′, abruptchanges in heat sink thicknesses, or other thickness profiles, dependingon the heat extraction profile desired. Alternatively or in addition,the cladding 24 on any or all of the surfaces of the core 22 could bemade with various and/or varying thicknesses (not shown) to enhance orprofile heat extraction from the core 22.

It may be preferable for the heat sink 112 to comprise a material thathas a higher coefficient of thermal conduction than the core 22 in orderto avoid a problem of inability of the heat sink 112 to conduct heataway from the core 22 at least as fast as the heat is produced in thecore. However, a heat spreader can also be used beneficially as a heatsink, even though it may have a lower coefficient of thermal conductionthan the core 22, if such heat spreader can pull heat away from the core22 efficiently enough to keep the core 22 from overheating. Syntheticdiamond is a good heat spreader for this purpose.

As illustrated in FIG. 13, a broad heat sink 112 with a flat surface 114can be used to mount and support an elongated, multi-mode, rectangularwaveguide amplifier 100 as well as a number of laser diode pump lightsources 116, 118, 120, 122, 124, 126, 128. The top flat surface 114 ofthe heat sink 112 is coupled by thermal conduction to a flat bottomsurface of the rectangular waveguide 100. As mentioned above,quasi-one-dimensional waveguides or other rectangular waveguides with alarge aspect ratio of transverse widths or thicknesses, i.e., much widerin the direction of one axis 23 than in the direction of the other axis21 (see FIG. 5 and related discussion above), result in nearlyone-dimensional heat flow from the core 22 in the one or both directions(up and/or down) of the vertical axis 21 (FIG. 5) and, if clad, throughthe cladding 24. Therefore, such a broad aspect ratio is particularlybeneficial for thermal coupling of a broad, flat side 130 of thewaveguide 102 to the heat sink 112 for efficient dissipation of heatfrom the waveguide 102 to the heat sink 112. Likewise, the flat sides ofthe laser diode pump sources 116, 118, 120, 122, 124, 126, 128 areconducive to such efficient thermal coupling and heat dissipation to theheat sink 112. For a heat sink 112 on a non-clad amplifier core, such asthe unclad core 20′ of FIG. 12, an intervening layer of a heatconducting material (not shown) with a low enough index of refraction tonot interfere with the waveguiding of light in the core 20′ can be usedto conduct heat from the core 20′ to the heat sink 112. Such anintervening layer may comprise, for example, a fluoropolymer material ora silico-oxide material.

Also, the elongated, narrow sides of the waveguide 102 accommodateoptical coupling of wide-beam, laser diode pump sources 116, 118, 120,124, 126, 128 to the waveguide core 22 without significant light energylosses or need for focusing systems, although stacked laser diodes 122with condensing optics, such as lens duct 123, or micro-optic arrays canalso be accommodated, if desired, for more pumping power. The stackeddiodes 122 and condensing duct 123 are illustrated in FIG. 13 positionedat an end of the waveguide 102 for example only. Such stacked diodescould be used anywhere along any side or end and in any number desired.

Basically, the cladding 24 can be a material that is transparent to thepump light, as long as it has a lower index of refraction than the core22 material, is desirable, but not required, to confine light in thewaveguide according to well-known principles as described above. Thepump light can be injected exclusively into and trapped by the core 22.As shown in FIG. 13, laser diodes 116, 118, 120 coupled to one lateralside 132 of waveguide 102 can be offset in relation to the laser diodes124, 126, 128 coupled to the other side 134 of the waveguide in order tospread the pump light evenly along the whole length of the waveguide 102for more efficient absorption of the pump light in the core 22. Ofcourse, unstaggered pump diode mounting configurations are also possibleto increase the pump light population of the device, but they tend to beless energy-efficient.

An optional second heat sink 112′ illustrated in phantom lines in FIG.13 can be placed on the flat, top side 131 of the rectangular waveguide102 to further increase heat dissipation from the waveguide 102. Ofcourse, the heat sinks 112, 112′ can also be used with laser resonatorsas well as amplifiers to dissipate heat.

The heat sinks 112, 112′ can be passive or active. Passive heat sinksare preferably fabricated of one or more material that has high thermalconductivity, such as carbon—carbon composite, which has a thermalconductivity of 20 watts/cm·K. Active heat sinks, such as silicon orcopper micro/mini channel fluidic heat sinks are capable of extractingapproximately 1 kilowatt/cm² of thermal flux.

A rectangular, multi-mode self-imaging, waveguide with a clad, solid,dielectric, core comprising Nd:YAG and sized 200 μm×2 cm in an amplifieror laser resonator, according to this invention, can operate at pulseenergies of 100 mJ for 10 nanoseconds full width at half-maximum pulseswhile maintaining a safety margin of one-fourth to one-half the opticaldamage threshold. With proper thermal control, as described above, andcareful design and sizing of the self-imaging waveguide length, pulseenergies can be scaled up to 250 mJ, which is useable for a wide varietyof commercial applications.

A particularly attractive preferred embodiment is a sapphire (Al₂O₃)clad 24, one-dimensional rectangular self-imaging YAG waveguide core 22,as described above, with silicon or copper microchannel cooled heatextractors 112, 112′ with a plurality of side pump diodes, asillustrated in FIG. 13. The one-dimensional core 22 ensures a short heatdissipation path through the core 22 to the cladding 24. The sapphirecladding 24 has very high heat conductivity and a high damage threshold,as well as being a very high quality, optically transparent crystalmaterial, so it transmits pump light to the core 22. Also, the cladding24 is preferably no thicker than about 1 mm so that there is little, ifany, absorption of pump light energy in the cladding 24, and so that theheat conduction path through the cladding 24 to the heat extractors 112,112′ is minimal. As mentioned above, the silicon or copper microchannelcooled heat extractors 112, 112′ can reach 1,000 watts/cm² or more heatextraction.

The rectangular, multi-mode, self-imaging, waveguide for amplifier orlaser applications according to this invention can also have opticalfiber cladding 142, as illustrated in FIG. 14, since the cladding 142does not have to be rectangular as long as it surrounds the rectangularcore 140 and optically confines the light in the core 140, such as byhaving a lower index of refraction, as described above. The oppositeaperture or face of the optical fiber is not shown, but, again, would bepositioned to provide a waveguide length equal to WSIP×i for the reasonsdescribed above. Pump light can be injected into the cladding 142 topump the core 140. Input and output light beam coupling for amplifierapplications or resonator optics for laser resonator applications can bedescribed above.

A double-clad fiber optic amplifier or laser structure 150 is showndiagrammatically in FIG. 15, wherein the rectangular, multi-mode,self-imaging, waveguide core 152 is surrounded by a first cladding 154that carries pump light energy to the core 152. The first cladding 154is surrounded by a second cladding 156 to confine the pump light in thefirst cladding 154. Therefore, the first cladding 154 has an index ofrefraction less than the core 152 in order to confine the amplifiedlight in the core 152, and the second cladding 156 has an index ofrefraction less than the first cladding 154 to confine the pump light inthe first cladding 154. This “double-clad” configuration 150 isgenerally more desirable than the single-clad configuration justdescribed, because contact induced pump light losses can be practicallyeliminated. The pump light enters the core 152 at virtually all anglesallowed by cladding total internal reflection, thus providing efficientsaturation of the core 152 with pump light energy. Of course, the lengthof the waveguide 152 between its two faces (not shown) is equal toWSIP×i, as described above. Input and output light beam coupling foramplifier applications or resonator optics for laser resonatorapplications can be as described above.

In another rectangular, multi-mode, self-imaging, waveguide embodiment160, illustrated in FIG. 16, which waveguide 160 can be used accordingto this invention for both optical amplifier and laser resonatorapplications, the waveguide 160 comprises a core 162 comprising gainmedium sandwiched between two cladding layers 164, 166. Two dichroicmirrors or reflective coatings 168, 170 cover portions of the lateraledges 169, 173 of the waveguide 160, and the input beam 172 is directedat an angle into the first waveguide aperture or face 176 toward theopposite reflective coating 168, while pump light 171 is directedthrough the dichroic mirrors or coatings 168, 170 into the core 162.Pump light can also be injected directly into an end of the core 162, asshown diagrammatically at 171′. The beam 172 is reflected by coating ormirror 170, which also reflects it back toward mirror 168, etc.Consequently, the beam 172 propagates through the waveguide 160 in azig-zag path, which is much longer than a straight path through thewaveguide 160. Eventually, the beam emerges as an output beam 174 from asecond aperture or face 178 that is not covered by the reflector 168.The longer path of the beam 172 through the waveguide 160 allows moreamplification of the beam 172 and more extraction of energy fromwhatever pump light 171, 171′ or other apparatus or method (not shown inFIG. 11) is used to excite or pump the gain medium of core 172. In thiscase, the effective waveguide length between the first face 176 and thesecond face 178 is the length of the zig-zag path of the beam 172through the waveguide 160, not a straight line length from the firstface 176 to the second face 178. Therefore, the zig-zag length wouldhave to be equal to WSIP×i, according to this invention. This zig-zagembodiment 160 is particularly adaptable to a one-dimensional orquasi-one-dimensional multi-mode waveguide configuration, as describedabove. It could also be tapered, as explained elsewhere in thisspecification, and it could be unclad rather than clad, as explainedabove.

Another embodiment of amplifier or laser resonator according to thisinvention with a zig-zag optical path 175 through a one-dimensional orquasi-one-dimensional, multi-mode waveguide is the so-called “light box”embodiment 161, illustrated in FIG. 17, wherein one or both of thelateral edges 169, 173 are tapered inwardly toward the longitudinal axisZ of the waveguide 161. As a result, the zig-zag path 175 of the beam inthe waveguide has a progressively smaller incident angle β with theedges 169, 173, thus becomes more and more dense, until it reaches aterminal path segment 175′ at a maximum distance from the input aperture176, where it reaches a terminal path segment 175′ at a maximum distancefrom the input aperture 176, where it reverses itself and retraces itszig-zag path 175 back through the waveguide 161 to emerge as anamplified beam 174′ in the same path as the input beam 172. Therefore,the outlet aperture 178 of the FIG. 16 embodiment 160 is not needed inthis lightbox embodiment 161. There are several advantages in thislightbox embodiment 161, including more density in the zig-zag path 175in which the beam extracts pump light energy, especially as itapproaches the terminal path segment 175′, and such zig-zag path iseffectively doubled for even more energy extraction by its propagationin the reverse direction, back toward the inlet aperture 176. Therefore,extraction of pump light energy with the input beam 172 as it propagatesthrough the waveguide core 162′ to emerge as the amplified output beam174′ is very efficient. Of course, input/output optics (not shown inFIG. 17, but similar to those described above in relation to FIGS. 9,10, and 13), would be required for coupling input beam 172 into, andoutput beam 174′ out of the waveguide 161.

Also, the total optical path length 175 from the input aperture 176, tothe terminal optical path segment 175′, and back to the input aperture176 has to be WSIP×i for the input beam 172 profile to be preserved inthe amplified output beam 174′, as explained above. The pump lightsources can be positioned to couple pump light energy into any one ormore of the lateral sides 169, 173, as indicated by the arrows 171, 171″in FIG. 17, and/or into the end 177, as indicated by arrow 171′. Sincethe highest densities of the optical path 175 occur as it approaches theterminal optical path segment 175′, positioning the pump light sourcefor end pumping 171′ and/or for side pumping near the end 177, asillustrated by arrow 171″ can be particularly beneficial.

A portion of the benefits of the light box 161 of FIG. 17 could beobtained with the FIG. 16 embodiment 160, of course, by merelyreflecting the output beam 174 back into the waveguide 160, but it wouldnot get the benefit of the higher density portions of the zig-zag beampath 175 of the light box embodiment 161 in FIG. 17. More density may beobtainable in the waveguide 160 of FIG. 16, by adjusting the angle ofincidence, but that variation would not have the benefit of positioningthe higher density paths near one end for more efficient end-pumping171′.

In FIG. 17, both of the lateral edges 169, 173 are shown as straight andtapered at an angle α in relation to the longitudinal axis Z. Specificangles α and angles of incidence for a desired waveguide 161 and opticalpath 175 geometries will depend on wavelength of the light beam, indexof refraction, and other factors, but can be determined empirically ormathematically by persons skilled in the art, once they understand theprinciples of this invention. Also, one or both of them could be curvedor could have different portions tapered at different angles to achievethe same advantages and results.

Another zig-zag or “tilt” beam path embodiment, double-pass amplifier180 is illustrated in FIG. 18, wherein the input beam 182 is directedinto the end face or aperture 184 in an off-axis orientation, i.e., notparallel to the longitudinal axis Z of the waveguide core 188. Thewaveguide 190 is rectangular and multi-mode, preferably with across-section similar to that shown in FIG. 5 or FIG. 6, although thesquare cross-section of FIG. 7 would also work. Therefore, for purposesof illustration, but not for limitation, the waveguide 190 cross-sectionillustrated in FIG. 18 corresponds with a cross-section takensubstantially along the horizontal axis 23 in FIG. 5. Thus, theinterface 35 between the core 188 and lateral side cladding 192 in FIG.18 corresponds with the core/cladding interface 35 in FIG. 5, and theinterface 37 between core 188 and lateral cladding 194 in FIG. 18corresponds with the core/cladding interface 37 in FIG. 5. The top andbottom cladding cannot be seen in FIG. 18, but is there, as taught aboveand as shown in FIGS. 5–7. A lens system represented symbolically by thelens 196 is any suitable optical system that focuses the incoming beamin the off-axis orientation desired onto the end face or aperture 184.An image relay lens 197 can be used, if necessary, to focus andcondition the beam 182′. The core 188 and cladding have respectiveindices of refraction that confine the beam 182′ in the core 188, asexplained above, and a non-clad core described in relation to FIG. 12would also work for this embodiment 180. Pump light or energy,represented symbolically by arrows 30 can be provided in core 188 by anymethod or apparatus known in the art, such as the laser diodes shown inFIG. 12 for a dielectric core, electrically if the core is asemiconductor, and the like.

As the light beam 182 traverses the core 188 in the first leg 183 of thezig-zag pattern or path shown in FIG. 18, it is amplified by the pumplight or energy 30 in the gain medium of core 188 to emerge from endface or aperture 186 as a partially amplified beam 182′. The partiallyamplified beam is redirected by any suitable optical system, forexample, the pair of spectral mirrors 198, 199, back into the aperture186 for another pass through the core 188 for further amplification byextraction of pump light energy with the beam 182. The second pass isalso preferably, but not necessarily, oriented off-axis to traverse asecond leg 183′ of the zig-zag path, through the core 188. It ispreferable, but not necessary, for the second pass or leg 183′ of thebeam 182′ through the core 188 to follow a different path than the firstleg 183 in order to increase energy extraction from the core 188 as wellas to distribute the thermal load more evenly and thereby avoidlocalized extreme thermal gradients that can produce optical distortionsin the core, as described above. Therefore, the twice-amplified beam182″ emerges from the end face or aperture 184 in a different path thanthe incoming beam 182. Therefore, the end face 184 functions as both aninput aperture for input beam 182 and an output aperture for theamplified output beam 182″, and more complex input/output opticalsystems, such as the polarizing beam splitter and ¼-wave retarder inFIGS. 9 and 10, can be avoided.

If the input beam 182 is tilted from the longitudinal axis F of thewaveguide 180 by an angle of more than wavelength divided by aperture(λ/a) (i.e., the angular extent of the central diffraction lobe due tothe waveguide aperture), the re-phasing or self-imaging requires fourtimes the propagation distance as non-tilted beams due to brokensymmetry, and the injected and recovered beams can easily be separated.Again, it is not necessary for re-imaging to occur at the end face oraperture 186, as long as the total distance traveled by the beam in bothpasses through the core 188 is equal to WSIP×i at the end face oraperture 184.

Another amplifier embodiment 240 of the present invention shown in FIG.20, like other embodiments described above, utilizes a multi-mode,one-dimensional, rectangular, self-imaging, waveguide with a lengthequal to WSIP×i, but which is flared in the non-imaging (transverse)direction. A core 242, sandwich-clad by cladding 244, 246, is narrowerin transverse width, i.e., transverse to the longitudinal axis 243, atthe inlet aperture and 248 than at the outlet aperture end 250. The core242 is side-pumped by a plurality of laser diode pump sources 256, 258,260, 262, 264, 266, 268, 270, for example, preferably coupled throughanti-reflective (AR) coatings or by dichroic mirrors 252, 254 to lateralsides of the core 242 to accommodate double pass, multiple wavelength,pumping. This flared configuration is particularly adapted to provideincreasing cross-sectional areas of the core 242 as the input beam 272propagates through the core 242 and picks up more and more energy fromthe pump light to produce the amplified output beam 274. If desired, thelaser diode pump sources can be higher power lasers toward the outputaperture end 250, where the core 242 has larger cross-sections, ascompared to lower power lasers for those nearer the input aperture end248. Of course, the thermal management and heat dissipation materialsand components, super-Gaussian beams, and other features discussedabove, can also be used in this flared amplifier embodiment 240.

Wavelength and phase control for amplifiers and laser resonators can becontrolled in the rectangular, multi-mode, self-imaging, waveguides ofsuch amplifiers and resonators by utilization of diffractive modulators200, such as the liquid crystal modulator illustrated in FIG. 19. Astatic external grating, embedded photo-reactive grating, or any otherform of periodic phase or amplitude grating could be used instead of theliquid crystal modulator 200. The optical components for coupling lightinto and out of the waveguide 20′ are not shown in FIG. 19 in order toavoid unnecessary complexity in the drawing. The example diffractive,liquid crystal modulator 200 is illustrated diagrammatically on theunclad core 22 of the waveguide 20′ of FIG. 19, but it can also beapplied to clad amplifier and laser resonator waveguides describedherein. Essentially, there are many liquid crystal materials, such asnematic, smectic, cholesteric, or ferroelectric liquid crystals, thatcan be set up in such a was as to change index of refraction in responseto change of voltage across such liquid crystal materials, and change ofindex of refraction of a material that interfaces with the waveguidecore affects the light confinement effectiveness of the waveguide forvarious wavelengths of light. Therefore, with the liquid crystalmodulator 200 mounted on a surface 201 of the core 22, the effectiveindex of refraction of the liquid crystal material 202 adjacent the core22 can be varied by varying the voltage across the liquid crystalmaterial.

A liquid crystal modulator 200 can be constructed in many ways, as iswell within the capabilities of persons skilled in that art once theyunderstand the principles of this invention, but one such examplestructure is shown in cross-section in FIG. 19. Essentially, the liquidcrystal material 202 is placed between two transparent conductive oxidefilms 203, 204, which function as electric contacts on opposite sides ofthe liquid crystal material 202 and which are brushed in a manner thatimposes a directional alignment of liquid crystals that contact them.One transparent conductive oxide film 204 can be deposited on thesurface 201 of the core 22, and the other transparent conductive oxidefilm 203 can be deposited on a cover plate 207 of glass or othertransparent material. Lateral supports 205, 206 contain the liquidcrystal material 202 and support the cover plate 207.

In operation, a voltage applied across the liquid crystal material 202is preferably set at a level that makes the effective index ofrefraction of the liquid crystal modulator 200 at a value that confinesthe desired light wavelength λ in the core 22, but which couples outother light wavelengths, such as λ₁, λ₂, λ₃ in FIG. 19. If desired alight sampling window 208 can be provided to extract a sample of lightfrom the core 22 to monitor the intensity of light of different,unwanted, wavelengths, for example, λ₁, λ₂, λ₃. A wavelength monitor 209coupled to the window 208 can be used to monitor such other unwantedwavelengths λ₁, λ₂, λ₃, which are commonly produced by wider band gainmaterials, for example Nd:YAG. Such wavelength monitors are well-knownand commercially available, such as Fabry-Perot etalons, or can be madewith dichroic light filters and photodetectors to admit and detectcertain wavelengths λ₁, λ₂, λ₃, etc. With input from the wavelengthmonitor 209, a controller 211 can function in as feedback loop to varythe voltage across the liquid crystal material 202 in a manner thatoptimizes coupling of such unwanted wavelengths λ₁, λ₂, λ₃ out of thecore 22. By changing the index of refraction of the proximal modulator200 uniformly, rather than in a periodic fashion, phase shifting of theself-imaging waveguide 20′ output at high average powers can beimplemented, which has clear power scaling and performance advantagesover transmissive phase shifters in series with a self-imagingwaveguide. Additional modulators 200 can be added to the waveguide 20′on either the top side or the bottom side, or both, if desired.

The diffraction grating 200, however implemented, acts as a spectralfilter, either in transmission or reflection mode, with its spectralbandwidth and transmission determined by the grating shape, modulationdepth, length, and depth relief with respect to the self-imagingwaveguide 102′ walls. The wavelength-selective reflection andtransmission filter could also be implemented with gratings embedded inmore than one of the self-imaging waveguide 20′ walls. The grating canserve as wavelength-selective reflectors on waveguide stubs fromT-junctions and as wavelength-dependent phase shifters. The optical“stub” wavelength-dependent reflectors can also be used to constructwaveguide circulators in direct analog to the microwave circulators,which are well-known to persons skilled in the art.

It is also worth mentioning that squeezing a core of a self-imagingwaveguide can also phase shift light propagating therein, thus can beused with some applications of this invention. This technique works forboth hollow and solid, dielectric cores.

The power scalable devices, components, and methods described above inrelation to FIGS. 1–19 are primarily for amplifiers, including laserresonators, that are provided for creation of the high power beamsneeded for various applications, for example, the ladar ranging,targeting, or imaging system 300 in FIG. 1. However, the rectangular,multi-mode, self-imaging, waveguide principles described above can alsobe applied to passive transport of such high power beams to points oflaunch or application of such beams to industrial, medical, imaging,ranging, tracking, and the like, while maintaining desired beam quality,temporal and spatial coherence and profile, polarization, phasing, etc.To illustrate some of the capabilities and features of beam transportand delivery methods and apparatus based on rectangular, multi-mode,self-imaging, waveguide technologies according to this invention,reference is made to FIG. 21, which is an enlarged, perspective,diagrammatic view of portions of the high power optical system 300 fromFIG. 1.

The ladar beam generator 302 in FIG. 21 can include any laser amplifierand/or resonator equipment 301 that can produce a high power beam 337with a desired spatial profile, preferably a lower order super-Gaussianbeam produced with one or more of the laser amplifiers, including, butnot limited to, laser resonators, of this invention, as described above.Other components needed for a functioning ladar beam generator, such asfrequency and pulse controllers, pump light sources, beam sampling andheterodyne components, and the like, are well-known to persons skilledin the art and not part of this invention, thus are not shown ordescribed herein. A lens system for focusing the ladar beam 337 onto theaperture 323 is represented symbolically by the lens 303, but caninclude any appropriate optical system for coupling the ladar beam 337to the waveguide 322 in a manner that focuses the beam 323 at or insidethe aperture 323. Alternatively, while not shown in FIG. 21, the laserresonator and/or amplifier 301 can be fabricated as an integral part of,or inside of, the self-imaging waveguides 322, so that the laser beam337 is emitted by such a laser resonator and/or amplifier 301 inside theself-imaging waveguide.

The rectangular, multi-mode, self-imaging, waveguides 322, 324, 326,330, 334 are sized to transmit the high power laser beam 339 to thevarious launch apertures 306, 308, 312, etc., where it can be coupledout or launched for the desired ranging, targeting, imaging, or anyindustrial, medical, or other application, as illustrated symbolicallyby the beam 340 in FIG. 21. Backscatter wavefronts 341, which arereflected or scattered by a target or scene area (not shown in FIG. 21)illuminated by the beam 340, can be received back into the waveguide 326via the same launch aperture 306 or by other apertures into the samewaveguide or into different waveguides. A coupler 344 can also couplethe laser beam from one waveguide 324 into other waveguides 330, 334,and the like, as will be explained in more detail below.

To launch the beam 340 with the desired spatial profile, such as thepreferred, lower order super-Gaussian profile described above, thelaunch apertures 306, 308, 312, etc., should be positioned at a distanceWSIP×i from the initial focal plane, e.g., the inlet aperture 323 if thebeam 339 is focused at the inlet aperture 339, as explained above, sothat the beam being transported in the waveguides 322, 324, 326, etc.,re-phases at the launch aperture 306, 308, 312, etc. Because of theself-imaging or re-phasing characteristic of the rectangular waveguidegeometry, the aperture 323 and cross-sectional area of the waveguides322, 324, 326, etc., can be orders-of-magnitude larger than asingle-mode aperture area or cross-section of core, thus can transportand deliver much higher power beams than single-mode waveguides andstill deliver the desired beam profile, e.g., lower ordersuper-Gaussian, at the output or launch aperture, which is not possiblein conventional, multi-mode optical fiber waveguides with circular, ovalor other conventional core cross-sectional shapes. Such rectangularconfiguration results in the re-construction of the input spatialprofile, generally, or that of the input complex amplitude profile. Ifthe rectangular waveguide core is comprised of a gas, gases, air, orvacuum, and has highly reflective waveguide walls, then nonlineardistortions of the spatial, spectral, and temporal coherence due to heatbuild-up or thermal gradients can be largely avoided up to the breakdownlimits of the waveguide walls or cladding. Thus, for correspondingintensity limits, the guided wave, high power, beam transport systems ofthe present invention can handle optical powers that may beorders-of-magnitude larger than single-mode waveguide systems whilestill reproducing and delivering the same input beam or image at theoutput. The increase of power handling capability may correspond to theincrease in waveguide aperture area, as more particularly describedbelow.

The rectangular geometry of the waveguides 322, 324, 326, etc., alsopreserves beam polarization with either uniform or non-uniform indexprofiles in either transverse direction, as further described below.Such polarization preservation, without the need for additionalpolarization components or materials, is important for energyconservation and thermal management purposes, because such polarizationcomponents and materials in conventional, multi-mode, optical fiber beamtransport systems absorb substantial amounts of light beam energy, whichis largely converted to heat. Also, polarization preservation isimportant to many beam input/output systems, beam analysis systems, beamsteering, and efficient, sharp cutting applications, etc.

The high power, beam transport waveguides 322, 324, 326 of the presentinvention can be fabricated of various materials in accordance withtraditional techniques in the art, for example, fabricated as aflexible, hollow, rectangular duct or as a stiff, crystalline core thatis either clad or unclad as described above in regard to the laserresonator and amplifier embodiments of this invention. Hollow waveguidesof the present invention may be further formed, embossed, coated orotherwise fabricated with various coatings, and in some instancesfabricated with dielectric coatings, including reflective interiorcoatings, depending on the desired characteristics of the wave guide andthe particular application or applications to which the waveguide is tobe applied.

Hollow waveguide embodiments can function with some bending andtwisting, and still re-phase or re-image periodically. Such bending ortwisting can sometimes cause small perturbations of the periodicself-imaging properties, but corrections can be made so that they do notaffect the overall system behavior. For example, a twist can have anoptical effect similar to incorporation of a negative lens, while a bendcan have an optical effect similar to incorporation of a positive lens.However, compensation for those effects can typically be implemented byspherical and stigmatic optics at the input and output ends of theself-imaging waveguide.

An enlarged example of a hollow, rectangular multi-mode, self-imaging,waveguide beam transport device 210 in FIG. 22 is similar to thewaveguide portions 322, 324, 326, 330, 334 of FIG. 21 and isrepresentative of waveguide transport devices of this invention forpurposes of explanation. An input beam 212, represented as an arrow, isintroduced to an optical aperture, entrance face, or cross-section 214of the waveguide 210. In some embodiments, one or a plurality of lens(not shown in FIG. 22) or abutment joints may be used to input, inject,or otherwise introduce the input beam 212 to the interior region or core216. One or a plurality of lens (not shown in FIG. 22) or abutmentjoints may be used to output an output beam 212′ from the interiorregion or core 216 at the end face or output aperture 215. Embodimentsmay also provide a coupling of a waveguide of the present invention withanother waveguide for either input, injection, or other introduction ofan input beam 212 into, or an output beam 212′ from such otherwaveguide. If another waveguide is used to input a beam 212 into arectangular, self-imaging waveguide 210 of the present invention 212′from a waveguide or to receive an output beam 210 of this invention suchother waveguide may, but need not necessarily, comprise a rectangular,self-imaging waveguide. Abutment couplings, not shown in FIG. 22, canalso be used to couple beams 212 from an optical amplifier or laserresonator of this invention as described above into the multi-mode,rectangular beam transport waveguide 210. The waveguide interior or core216 is preferably hollow, as shown in FIG. 22 and filled with air or agas that has a high breakdown threshold, such as helium, or it can beevacuated. Alternatively, the core 216 could comprise a solid or liquid(not depicted in FIG. 22). However, for low dispersion, high or veryhigh power beam transport applications, a hollow core 216 is best. Thewalls of the waveguide 212 can be actively cooled with an air or liquidcooling medium (now shown), if necessary, for high or very high powerbeam transport applications.

The multi-mode waveguide re-phases or self-images the spatial profilebeam 212 at the determined self-imaging period (WSIP) in the mannerpreviously described. The wave guide length is chosen such that theoptical path length is an integral multiple of the wavelengthself-imaging period, i.e., WISP×i, for re-phasing or re-imaging theinput beam 212 profile at the output aperture 215, as also explainedabove. The beam 212 can be coupled, injected, or otherwise introducedinto the waveguide 210 to travel along a central propagation axis Z, andexits the waveguide at an exit face or cross-section 215 at an oppositeend of the waveguide or through wall launch apertures 306, 308, 312, asdescribed above. However, as an alternative, the input beam 212 can bedirected to travel at a non-zero angle relative to the central axis 217of the waveguide 210, in which case the optical path or effectivewaveguide length WSIP×i will differ from the length of the waveguide210, as explained above in relation to FIGS. 16, 17, and 18. However, itmay be noted that indices of refraction for solid, dielectric cores aregreater than one, and, for some dielectric materials, can besubstantially greater than one, e.g., zinc selenite, which has an indexof refraction greater than 2.0 at 1.06 micron wavelength. Therefore, thenumerical aperture, thus also angle of acceptance, is greater for solid,dielectric cores than for hollow core waveguides. Consequently, solidcore waveguides can be more compact, but hollow core waveguides for highpower beam transport have the advantages of minimal dispersion, minimalnonlinear optical distortion, minimal heating, and excellent mechanicalflexibility for routing around curves and through places where availablespace is limited.

Furthermore, the hollow core 216 is surrounded by walls or cladding toprovide optical confinement of the beam 212 in the hollow core 216. Theoptical confinement can be provided by reflective cladding walls 220 orby other techniques, such as internal reflection from cladding layers220 that have a lower index of refraction than the core 216 material. Ifthe core 216 is a solid with a high enough index of refraction, such asphosphate glass, no cladding 216 is necessary, as described above inrelation to FIG. 12. Walls or cladding 220 may have a shape distinctfrom the cross-sectional shape or aperture of the waveguide, forexample, as shown by the optical fiber in FIGS. 14 and 15. Additionalregions exterior to the cladding may be provided in some embodiments toincrease structural robustness.

Cladding 220 may form a reflective region, and some embodiments mayprovide cladding composed of a medium of a lower index of refraction ascompared to core 216, to provide for total-internal-reflection, a mediumof intrinsic reflectivity at the propagation wavelength of a beam, suchas metal, or one or more dielectric coatings that reflect the beam of aparticular wavelength, among others. However, hollow, passive systemsmay, in some embodiments, avoid dielectric coatings such thathydroscopic delinquence or other effects capable of damaging thewaveguide 210 can be avoided.

Reflections of rays or modes of the beam 212 propagating in thedirection of the longitudinal waveguide axis Z, occur at opposingcore/cladding interfaces explained above. The waveguide 210 can beshaped and sized to provide a high power beam transport that ismulti-mode in one transverse direction or in two dimensions, asdescribed above.

The effects of self-imaging waveguide deformation are largely the samefor dielectric and hollow core waveguides, aside from the higher indexof refraction in the dielectric self-imaging waveguides. A hollow ordielectric ribbon waveguide that is bent, twisted, and/or buckled isoptically equivalent, in the first approximation, to a straightwaveguide of ideally flat walls, which is occupied by an effectiveoptically inhomogeneous index of refraction profile. These waveguidedistortions result in phase distortions of the propagating modes orself-imaging wavefronts. While sharp distortions result in propagationlosses, their effects within certain limits can be acceptable. Theanalytic expressions for wavefront changes due to bending, buckling,and/or twisting are quite complex and are not necessary for anunderstanding of this invention, because it is quite easy to determineempirically when such distortions become too sharp for self-imagingpropagation or when propagation losses become unacceptable forparticular waveguide sizes, wavelengths, and applications.

Only a bend has a simple analytic form, and only under gentle bendconditions. Bending corresponds to linear gradients of an effectiveindex across either of the transverse coordinates, i.e., perpendicularto the optical axis of the self-imaging waveguide 210. Therefore, suchbending leads to an effective linear tilt of the wavefront inside of theself-imaging waveguide 210, but that tilt is only acceptable up to thebend radius limits shown below. Specifically, pure bending notaccompanied by twists or buckles has a critical bending radius limitgiven byρ≧α³/λ²,where α is the aperture height and λ is the wavelength. The bendingresults in tilting the wavefront of the propagating wave and results inmore than approximately quarter wave of r.m.s. wavefront error for ahalf circle bend, which is fiducially referred to as unacceptablewavefront error.

Buckling has reference to the dependence of waveguide height on localcoordinates. Buckling can also be modeled as a variation in theeffective refractive index, whether the self-imaging waveguide 210 ishollow or not. Local variations of the ribbon thickness relate toproportional variations of the refractive index. Buckling the ribbonwaveguide results in a focusing effect, in the contrast to twisting,which leads a defocusing effect. The strength of focusing stronglydepends on the index number of the classical eigenmodes.

Twisted portions of a rectangular self-imaging waveguide 210 inducespath differences for the propagating rays and results in defocusing. Theeffects of twists and buckling in a ribbon waveguide 210 are of theopposite sign, which means that there exists a possibility of cancelingone effect by the other. For example, if the twist rate is less than 30degrees over one meter length in a waveguide 210 that was a rectangularcross-section of 1 cm×0.1 mm, the wavefront deformations remained lessthan 0.1 wave r.m.s. at 2 micron wavelength. On the other hand, if thetwist rate was increased to 90 degrees per meter, the wavefront splitsinto more than one spatial mode, which can be detrimental toapplications requiring spatial coherence.

The sidewall launch apertures 306, 308, 312, etc., of FIGS. 1 and 21 forextracting or coupling portions or all of the light energy out of, orinto, the waveguides 322, 324, 326, 328, 330, 334, etc., can be providedand constructed in a number of ways, including technologies that canchange the index of refraction of the cladding of the waveguide at thelocation of the aperture in a manner that allows light in the waveguideto leak out or escape the core. One example sidewall launch aperture 306is illustrated diagrammatically in longitudinal cross-section in FIG.23, wherein a sidewall window 380 comprising an electrically addressableliquid crystal material 382 can be actuated to change the effectiveindex of refraction of the window 380 to enable or disable evanescentleakage of light energy out of the core 384 of the sidewall launchaperture 306. The sidewall launch aperture 306 can be constructed, forexample, with a body 385 shaped on its interior to enclose arectangular, multi-mode, waveguide core 384 with dimensions to match thecore 216 of the waveguide 326, and it can include a reflective interiorcoating 389, if necessary, to ensure the necessary reflectivity topropagate the light 339 through the core 384. The liquid crystalmaterial 382 is sandwiched between a transparent substrate 386 and atransparent cover plate 388, each of which has a transparent conductiveoxide layer 390, 392 brushed to impose a boundary layer crystalorientation, as is well-known and understood by persons skilled in theart of liquid crystal light modulators. The liquid crystal material 382has an index of refraction that varies as a function of voltage appliedacross the liquid crystal material 382. The voltage is applied via thetransparent conducting oxide layers 390, 392, which are convected to avoltage controller (not shown) by wires 393, 394 or other conductors.Myriad suitable voltage controllers are available or well within thecapabilities of persons skilled in the art and are not part of theinvention, thus need not be described here. Contact posts 396, 398 canbe used to connect the wires 393, 394 electrically to the transparentconductive oxide layers 390, 392, and, if the body 385 is plastic orsome other non-conductive material, no further electrical insulation isneeded.

By setting the voltage across the liquid crystal material 382 at a valuewhere the effective index of refraction of the window 380 causes Talbotrefraction containment of virtually all of the light of a particularwavelength of the light beam 339 in the core 384, such light does notescape through the window 380. However, a suitable grating, such asspatially periodic change of voltage across the liquid crystal material382, causes a change in index of refraction of the window 380 to allowevanescent leakage or coupling of light energy 340 out of the sidewallaperture 308, where it can be focused or columnated in a beam withappropriate optical components (not shown) for a particular application,such as ranging, targeting, imaging, cutting, and the like. Suchfocusing or columnating optical components are not part of thisinvention, but can be easily designed and implemented by persons skilledin the art.

For launching a beam 340 with the desired spatial profile that isinjected or focused into the waveguide, for example, a lower ordersuper-Gaussian beam, as described above, the sidewall launch aperture306 should be positioned at an integer multiple of the waveguideself-imaging period (WSIP×i) so that the window 380 is aligned with animaging plane, as also described above and illustrated in FIG. 21.Surprisingly, but as has been shown in comprehensive laboratorymeasurements, a wall out-coupling grating or aperture 306, as describedabove, as long as one-fourth of a WSIP still out-couples a neardiffraction limited ( 1/10 wave peak to valley) wavefront Gaussianprofile laser beam. However, the fractional out-coupling of a wallcoupler, such as the grating or aperture 306, needs to remain low, suchas about ten percent or less per ¼-Talbot period, in order not to overlydistort the wavefront of the remaining energy flowing in theself-imaging waveguide. The voltage across the liquid crystal material282 can also be varied to couple out selected proportions of the lightenergy 339 anywhere in the range extending from none to all of suchlight energy. Since the window 380 is controlled by voltage, asdescribed above, it is very conducive to convenient remote control.

An alternate, grating window, embodiment 400 can be used in place of thewindow 380 in the sidewall launch aperture 306 to provide steering ofthe output beam 340, as illustrated diagrammatically in FIG. 1, and as340′ and 340″ in FIGS. 24 a–c. With initial reference to FIG. 24, asmall portion of a grating window 400 is shown in an enlargedcross-section. Essentially, the grating window 400 can be anydiffraction outcoupler embodiment, as explained above, but in a gratingformat. Fixed and programmable diffractive incouplers/outcouplers can beachieved via processes, such as (i) liquid crystals; (ii) semiconductorgratings—either etched or electrically programmable; (iii)photorefractive gratings—either electrically or optically written; (iv)photochromic and/or electrochromic gratings; or (v) etched reliefgratings. Moreover, the diffractive grating structure can be curved,linear, chirped, or any combination of these features.

An example grating window 400 is similar to the window 380. It hasliquid crystal material 382 sandwiched between the substrate 386 and thecover plate 388, but the transparent conductive oxide (TCO) layer oneither the cover plate 388 or the substrate 386 is divided into aplurality of juxtaposed, narrow, elongated TCO strips or contacts 402.Each of the strips 402 is separated from each other by a dielectric orelectrically insulative material 404 and is electrically addressableindividually, for example, via individual wires or conductor traces 406connected to respective ones of the TCO strips or contacts 402.Therefore, different voltages can be applied across correspondingportions of the liquid crystal material 382 that are juxtaposed to therespective TCO strips 402 and thereby vary index of refraction of suchrespective, juxtaposed portions of liquid crystal material 382individually. Again, as explained above, such change of index ofrefraction of the liquid crystal material 382 effectively turns on andoff light energy leakage or transmission through the window 400, and theability to turn alternate individual strips of the window 400 on and offeffectively creates an optical grating. The density of the juxtaposed,alternate on and off, strips of the window 400 determine the angle ordirection at which the output light is diffracted or propagated by thegrating. Since the TCO strips 402 can be on the order of severalmicrometers wide and are addressable individually, the effective gratingdensity of window 400 can be set as desired for a wide range of outputlight diffraction angles or propagation directions. For example, onegrating density can be provided by turning on and off alternate bands offive (5) TCO strips 402 apiece, and another grating density can beprovided by turning on and off alternate bands of ten (10) TCO strips402 apiece. Other patterns or groupings of TCO strips 402 create gratingcharacteristics and densities as desired or needed. The particulargrating line densities needed for particular desired angles orpropagation directions depends on the particular wavelength of the lightand diffraction indices of materials being used, but they can bedetermined empirically and/or analytically by persons skilled in the artwith well-known technologies and formulae.

As illustrated in FIG. 24 a, the light energy of beam 339 in therectangular waveguide of the sidewall launch aperture 306 (FIG. 23), isincident on the grating window 400. The arrows 339 are only generallyrepresentative of incident light energy on the grating window 400 andare not intended to indicate any particular beam 339 propagation mode orray direction in the waveguide. However, the arrows that are indicativeof the light energy in output beam 340 in FIG. 24 a do illustrate one ofmany possible output diffraction angles or beam propagation directions.In the illustration of FIG. 24 a the selected grating window densitypropagates the output beam 340 at an angle about perpendicular to thegrating window 400. Two different propagation directions of beam outputs340′, 340″ produced by two other grating window 400 densities areillustrated in FIG. 24 b and FIG. 24 c.

While not illustrated, it is worth noting that the individual TCO strips402 could be curved to also provide grating induced focusing of theoutput beams 340, 340′, 340″. Also, two or more grating windows, such aswindow 400, can be stacked or cascaded, one on the other, for severaldesirable effects. For example, two grating windows 400 stacked withtheir TCO strips 402 extending in the same direction can effect moreoutput beam angular adjustment capabilities in one direction. On theother hand, stacking two grating windows 400 with the TCO strips 402 ofone of the windows 400 oriented perpendicular to the TCO strips 402 ofthe other window 400 provides beam output angular adjustmentcapabilities in two directions, as long as the appropriate polarizationsare presented to the gratings, either by intrinsic self-imagingwaveguide propagation or by use of appropriate wave retarder plates, asis well understood and within the capabilities of persons skilled in theart.

A liquid crystal window 420, illustrated in FIG. 25, which is similar tothe window 380 shown in FIG. 23, is used in the junction aperture 344 toselectively couple light energy from the beam 339 in rectangularwaveguide 324 into one or both juxtaposed rectangular, multi-mode,self-imaging, waveguides 330, 334. The liquid crystal window 420 ispositioned between two juxtaposed, rectangular waveguide chambers 410,412 of the junction aperture 344. The liquid crystal window is actuatedto function essentially the same as the window 380 of FIG. 23, exceptthat, instead of light energy leaking or escaping through the window 420into free space, it is captured by the second waveguide chamber 412 forpropagation either in waveguide 330 or in waveguide 334. Essentially,the light energy from beam 339 coupled into, and captured by, the secondwaveguide chamber 412 and is then guided to propagate into one or bothof the rectangular, multi-mode, self-imaging, waveguides 330, 334.

While it is not essential, it is preferred that the window 420 bepositioned in a re-phasing or re-imaging plane, i.e., at an integralmultiple of the waveguide self-imaging period (WSIP×i) from the inputaperture 323, as shown in FIG. 21. Positioning the window 420 in otherlocations may cause some undesirable perturbations in the beam profile.

A switch aperture 430, shown in FIG. 25, illustrates diagrammaticallythe use of two liquid crystal windows 432, 434 for selectively switchinglight energy 436 from or to an input rectangular, multimode,self-imaging, waveguide 348 into or from one or the other or both of twooutput rectangular, multi-mode, self-imaging, waveguides 440, 442. Theswitch aperture 430 has three juxtaposed, rectangular, multi-mode,waveguide chambers 444, 446, 448, with central, input chamber 446separated from one output chamber 444 by one liquid crystal window 432and separated from the other output chamber 448 by the second liquidcrystal window 434. The liquid crystal windows 432, 434 aresubstantially like the liquid crystal windows 380, 420 in FIGS. 22, 24and are individually addressable, electronically, to couple light energyout of the central input chamber 446 into output chambers 444, 448, asdesired.

While the beam sidewall launch and beam coupling from or between therectangular multi-mode, self-imaging, waveguide, electronically, lighttransport systems of this invention are shown in FIGS. 22–25 asimplemented by liquid crystal window apertures, other mechanisms anddevices could be used to perform these functions within the scope of theinvention. For example, other embodiments of sidewall launches andcouplings into other waveguides consistent with the present inventioninclude, inter alia: grating technologies; diffraction grating; prismapertures; grisms (gratings and prisms, such as grating etched on faceof prism); prism evanescent wave coupling, microelctro-mechanicalapertures; aperture-window technology, generally, and arrays thereof.Sidewall launches may be particularly applicable to systems providingone or more potentially desirable features such as synthetic aperture,distributed aperture, beam forming, beam steering, and power sampling,among other features. Sidewall launches may be provided as coherent orincoherent sources, coherent sources particularly beneficial in imagingapplications, for example, and incoherent sources particularlybeneficial in laser medium pumping and radiometric applications.

In applications where even more power is required than can betransported and delivered by one rectangular, multi-mode, self-imaging,waveguide, e.g., waveguide 214 in FIG. 22, two or more rectangular,multi-mode, self-imaging, waveguides can be stacked together in an arrayto transport and deliver two or more complimentary, phase-matched beams.For example, the stacked array 450 of three rectangular, multi-mode,self-imaging waveguides 452, 454, 456 shown in FIG. 27, can be used totransport and deliver three, phase-matched beams 460, 462, 464 to acommon exit plane, i.e., the plane of the three exit apertures 472, 474,476. If the beams 460, 462, 464 are phase-matched with each other andfocused with a desired spatial profile, for example, lower ordersuper-Gaussian, at the respective co-planar entrance apertures 466, 468,470, the spatial profile of each beam 460, 462, 464 re-images at therespective exit apertures 472, 474, 476, provided that the exitapertures 472, 474, 476 are positioned at a distance of WSIP×i from theentrance apertures 466, 468, 470. The phase-matching of the beams 460,462, 464 can be accomplished as described above with either in-line,electro-optic, phase modulators, or, in the case of hollow self-imagingwaveguides, small piezo electric transducers producing small amounts ofcompression of the waveguide to introduce a uniform piston phase shiftacross the self-imaging waveguide aperture without appreciably alteringthe wavefront, or in other ways that are known to persons skilled in theart.

Any number of rectangular, multi-mode, self-imaging waveguides can beincluded in an array, as illustrated, for example, by the fiber-encasedarray 480 in FIG. 28. Such an array 480 can be used to transmit morepower than can be transmitted in one of the waveguides in the array.However, as also illustrated in FIG. 28, such an array can be used in alens spatial multiplexing application in which individual components ofan image or other data are produced and optically coupled or transmittedsimultaneously by a transmitter array 482 into the respective inputapertures 484 for transmission to output apertures 486 aligned with adetector array 488, on which the image is reassembled.

A sparse and/or synthetic aperture system 490 comprising a plurality ofrectangular, multi-mode, self-imaging, waveguides which has one or morebeam launch apertures 500, 502, 504, 506, 508, 510, is shown in FIG. 29.Light energy from the respective beams 512, 514, 516 is coupled,selectively, out of the waveguides 492, 494, 496 by launch apertures500, 502, 504, 506, 508, 510, which may utilize addressable, diffractiongratings 400, as described above in relation to FIGS. 24 a–c. In otherwords, the launched beams 512, 514, 516, 518, 520, 522 can be steeredand focused by the apertures 500, 502, 504, 506, 508, 510 to a commonpoint 524 or in any other desired directions. With appropriate phaseshifts between the wall in-couplers or out-coupler, synthetic aperturetechniques may be particularly useful in providing, for example,multiple-meter class apertures from launch and recovery systems,providing high resolution target ranging, velocity, and imaging results,particularly from moving platforms, such as may be applied in aeronauticand space applications, among others. In other wards, if theself-imaging waveguides are utilized in a ladar (laser radar) system,for example, back-scattered spherical or plane waves 511 for the scene524′ would be collected through micro-Doppler and range-tagged echos toallow image formation from the synthetic aperture over the full extentof the synthetic aperture baseline. For example, if the launched beams512, 514, 516, 518, 520, 522, are co-phased, i.e., all in phase witheach other, and if they are steered and/or focused to illuminate alarger area scene 524′, the combination of the multiple apertures 500,502, 504, 506, 508, 510 pick up the back-scattered waves 511, which aremuch higher energy, thus easier to detect and process for image contentfrom the scene 524′ than the back-scattered energy would be from onlyone of the beams 512, 514, 516, 518, 520, 522.

The sidewall launch beam forming and steering could also be provided byother diffractive or refractive techniques, gratings, prism apertures,and in some embodiments prism evanescent wave coupling,microelectro-mechanical apertures, aperture-window technology,generally, and arrays thereof, among other aperture technologies knownin the art, once the principles of this invention are understood. Also,multiple grating periods for one or a plurality of exit surfaces, facesor apertures, could be used for special application such that multiplebeam forming and steering from one exit surface, face, aperture orlaunch may occur either singly or in combination. Additionally, suchbeam forming may be applicable to systems wherein the diversion of powerfrom the waveguide is desirable, as in power monitoring of the waveguideand in power splitting techniques, generally.

Beam combining or power combining can also be implemented withrectangular, multi-mode, self-imaging, waveguides according to thisinvention, as illustrated in FIGS. 30 and 31. As shown in FIG. 30,multiple branches of such rectangular input waveguides 532, 534 can becoupled into an input aperture of an output or combiner rectangularwaveguide 540 to form a beam combiner 530. To avoid an unnecessarilycumbersome description, this explanation has only two beams 546, 548,and two corresponding waveguides 352, 354 being combined, but any numberof beams and waveguides can be combined in this manner at a common plane533. A first light beam 546 is focused into the input aperture 536 witha desired spatial profile, e.g., lower order super-Gaussian, to bepropagated through the first rectangular waveguide 532 to a beamcombining phase 532, which is at the entrance aperture of the outputwaveguide 540. At the same time, a second light beam 548, which ispreferably the same wavelength and phase-matched to the first beam 546,is focused into the input aperture 538 with a desired spatial profile.The length of each input waveguide is preferably WSIP×i, as explainedabove, so that both input beams re-phase or re-image at the beamcombining plane 533. The respective spatial profiles of the input beamscan, but do not have to, be identical. It is preferred to provide beams546, 548 with respective spatial profiles that will combine at plane 532to form a desired spatial profile, e.g., lower order super-Gaussian, forthe combined output beam 550, which will re-phase or re-image at theoutput aperture 542 if the length of the output waveguide 540 is WSIP×i,as explained above. (Of course, because the output or combiner waveguide540 has a larger cross-sectional area and waveguiding width than theinlet waveguides 532, 534, the WSIP for the combiner waveguide 540 isdifferent than the WSIP for the inlet waveguides 532, 534.) Therefore,the output beam 550 has the combined power of input beams 546, 548 withthe desired spatial profile.

In a modified beam combiner embodiment 530′, shown in FIG. 31, the inputwaveguides 532, 534 and output waveguide 540 are joined at plane 532 byrespective, adiabatically tapered portions 554, 556, 558. However, whenthere are such tapered portions 554, 556, 558, the WSIP varies, andthere is no integer multiple i that describes where re-phasing orself-imaging occurs. Therefore, to place a re-phasing or self-imagingplane at the outlet aperture 542, the self-imaging period distance D_(w)may have to be determined empirically.

The following table illustrates the various combinations of structures,features and attributes of waveguides (WG) that can be used in variousembodiments of this invention:

Invention aspect Claim Options Specific examples WG Aperture GeometryRectangular □ Quasi-One-dimensional □ Square WG Core Medium Gas □ SF₆Liquid* Laser dye in solution Doped and Undoped Dielectric □ CrystallineSolids, semiconductors □ Poly-crystalline □ Amorphous (Glass) WGCladding Medium Metal (conductor) □ Sold metal □ Metal coated Dielectricwith index of Liquid crystal grating refraction lower than arrays,homogeneous core index slabs, gradient index slabs, photo- refractivematerials, doped gain media, nonlinear optical materials Dielectriccoatings Fluoride and oxide single/multilayer dielectric coatings WGStructure Stiff □ Wafer Flexible □ Optical Fiber WG Length Optical pathlength is an integer multiple of the imaging period or fractionalre-phasing distances for beam splitting Active Operation (passiveOptical amplifier (see options below) options plus beam modification)Electro-optical modulator, □ Phase modulator degenerate and non- □Polarization modulator degenerate wave-mixing for □ Communications phaseor amplitude encoding for wavelength modulation, beam divisionmultiplexing, combination and energy PSK, ASK, QSK, etc. transferformats *Not a common configuration

Optical Amplifier General Example Specific Example Core Host Medium (anyGlasses □ Phosphate laser gain medium) □ Silicate Crystalline, quantumwells □ Garnet (YAG, . . .) □ Fluoride (YLF, . . .) □ Sapphire □ Oxides□ Germanites □ Chalcogenides □ Chlorides □ Apatites □ Elementalsemiconductors and compound stoichiometric and non-stoichiometric semi-conductors, quantum wells, quantum cascades, and all forms ofheterojunctions Gas □ CO₂ Solid Core Active Ion Rare Earth □ Nd (anylaser active ion) □ Er □ Yb Transition Metal □ Cr □ Ti Pumping Scheme(any laser Optical □ Flashlamp pump scheme) □ Laser □ Laser diode array□ Solar illumination Electrical □ Semiconductor diode WG amplifier/laserOptical/Electrical Pumping Longitudinal □ Preferable for any WG Geometryaperture geometry Transverse Preferable for one-dimensional WG alongwide transverse axis □ Face pumping may be useful for semiconductorlasers and control beams for dynamic wave mixing and gratings.Application specific Component of laser □ WG comprises entire laser □ WGamplifier is component of laser system Image Amplifier Endoscope, ladarpreamplifier

The foregoing description is considered as illustrative of theprinciples of the invention. Furthermore, since numerous modificationsand changes will readily occur to those skilled in the art, it is notdesired to limit the invention to the exact construction and processshown and described above. Accordingly, resort may be made to allsuitable modifications and equivalents that fall within the scope of theinvention. The words “comprise,” “comprises,” “comprising,” “include,”“including,” and “includes” when used in this specification are intendedto specify the presence of stated features, integers, components, orsteps, but they do not preclude the presence or addition of one or moreother features, integers, components, steps, or groups thereof.

1. An amplifier system for producing a high power laser beam, comprising: a multi-mode, self-imaging, waveguide having a core of solid gain or mixing medium with a rectangular cross-section, and cladding material that has a coefficient of thermal conduction, interior cladding surfaces abutting opposite, waveguiding surfaces of the rectangular core, and exterior cladding surfaces that are opposite the interior cladding surfaces; a beam input coupling system capable of providing a desired spatial phase profile of the laser beam at an entrance aperture of the core to propagate the laser beam into the waveguide; a pump light source coupled into the waveguide core medium to propagate pump light energy into the core medium to be extracted by the laser beam; a beam output coupling system capable of coupling an output beam from the core at a plane where the beam propagating in the core re-phases into the desired spatial phase profile; and a heat sink positioned adjacent and in contact with an exterior surface of the cladding material.
 2. The amplifier system of claim 1, wherein the heat sink has a coefficient of thermal conduction that is higher than the coefficient of thermal conduction of the cladding.
 3. The amplifier system of claim 1, wherein the heat sink comprises a heat spreader, which has a coefficient of thermal conduction lower than the coefficient of thermal conduction of the cladding.
 4. The amplifier system of claim 1, wherein at least one exterior surface of the cladding material is flat and wherein the heat sink has at least on flat surface that is positioned in contact with the flat exterior surface of the cladding.
 5. The amplifier system of claim 4, wherein the core is a one-dimensional, self-imaging, waveguide core, and wherein the cladding abutting one of the waveguiding surfaces has a heat sink with a flat surface abutting a flat exterior surface of the cladding and the cladding abutting the opposite one of the waveguiding surfaces also has a heat sink with a flat surface abutting a flat exterior surface of the cladding so that heat flow through the core to the cladding is substantially one-dimensional.
 6. The amplifier system of claim 4, wherein the heat sink is wider than the waveguide so that the flat surface of the heat sink extends laterally outward from the cladding, and wherein the pump light source includes at least one laser diode with a flat exterior side, said laser diode being positioned to couple light energy produced by the laser diode into a lateral side of the waveguide and with the flat exterior side of the laser diode in contact with the flat surface of the heat sink.
 7. The amplifier system of claim 4, wherein the heat sink has a uniform thickness.
 8. The amplifier system of claim 4, wherein the heat sink has a varying thickness for maintaining a desired temperature profile in the waveguide.
 9. The amplifier system of claim 6, wherein the pump light source includes a plurality of laser diodes with flat sides mounted on the flat surface of the heat sink and distributed spatially along opposite lateral sides of the waveguide in positions to couple light from the laser diodes into lateral sides of the waveguide.
 10. The amplifier system of claim 1, wherein the heat sink is passive.
 11. The amplifier system of claim 10, wherein the heat sink comprises carbon—carbon composite.
 12. The amplifier system of claim 1, wherein the heat sink is active.
 13. The amplifier system of claim 12, wherein the heat sink comprises a copper micro/mini channel fluid heat sink.
 14. The amplifier system of claim 1, wherein the core medium has a coefficient of thermal expansion and the cladding material has a coefficient of thermal expansion that is not more than twenty percent less than the coefficient of thermal expansion of the core material.
 15. The amplifier system of claim 14, wherein the cladding material is capable of bonding to the core material.
 16. The amplifier system of claim 15, wherein the core material comprises YAG and the cladding material comprises sapphire (Al₂O₃).
 17. The amplifier system of claim 15, wherein the cladding material has a thickness of no more than 1 mm.
 18. The amplifier system of claim 15, wherein the difference between heat conductivity of the core material and heat conductivity of the cladding material is no more than twenty-five percent of the heat conductivity of the cladding material.
 19. Laser amplifier apparatus, comprising: a multi-mode, rectangular, self-imaging, waveguide comprising a core of optical gain or mixing medium with a rectangular cross-section, opposed top and bottom surfaces, and opposed left and right lateral surfaces, reflectors adjacent the left and right lateral surfaces, an inlet aperture, and an outlet aperture; a pump light source coupled optically into the core; and an optical system positioned to couple an input laser beam with a desired spatial profile to the input aperture at an angle that propagates the laser beam to reflect off the reflectors in a zig-zag path in the core medium to the outlet aperture positioned in a re-imaging plane where the beam re-phases into the desired spatial profile, wherein the pump light source is coupled optically into a portion of the core where the optical path has a higher density than another portion of the core.
 20. The laser amplifier apparatus of claim 19, wherein the zig-zag path from the inlet aperture to the outlet aperture has a length equal to WSIP×i.
 21. The laser amplifier apparatus of claim 19, wherein the input beam has a first wavelength and the pump light has a second wavelength, the reflectors are dichroic mirrors that reflect light having the first wavelength and transmit light having the second wavelength, and the pump light source is positioned adjacent the left and right lateral surfaces to direct pump light through the dichroic mirrors into the core medium.
 22. The laser amplifier apparatus of claim 19, wherein the core has a first end face and a second end face at respective opposed ends of the core, the inlet aperture and the outlet aperture are both at the first end face, and the optical system also includes a reflective component adjacent the second end face positioned to redirect the beam, emerging from the second face after propagating through a first leg of the zig-zag path, back into the second face to propagate through a second leg of the zig-zag path to the outlet aperture.
 23. The laser amplifier apparatus of claim 22, wherein the reflective component adjacent the second end face is positioned to redirect the beam back into the second face in an orientation that causes the second leg of the zig-zag path to propagate through some portions of the core that are not occupied by the first leg of the zig-zag path.
 24. The laser amplifier apparatus of claim 20, wherein the outlet aperture is the same as the inlet aperture and at least one lateral edge is tapered toward a longitudinal axis of the core medium so that angles of incidence of the laser beam to the reflectors become smaller and density of the zig-zag path becomes greater as the beam propagates through the core medium until it reaches a terminal path segment at which the beam reverse propagates back through the zig-zag path to the inlet and outlet aperture.
 25. The laser amplifier apparatus of claim 24, wherein the tapered lateral edge is straight.
 26. The laser amplifier apparatus of claim 24, wherein the tapered lateral edge is curved.
 27. Laser amplifier apparatus, comprising: a multi-mode, one-dimensional, rectangular, self-imaging, waveguide including a core with a length equal to WSIP×i and which is flared outwardly in a non-imaging, transverse, direction so that the core has increasingly larger rectangular cross-sections from an inlet face at one end of the core to an outlet face at an opposite end; and a pump light source coupled optically to the core.
 28. The laser amplifier apparatus of claim 27, including cladding on waveguiding surfaces of the core.
 29. The laser amplifier apparatus of claim 27, wherein the pump light source includes a plurality of laser diodes distributed along at least one lateral side of the waveguide.
 30. The laser amplifier apparatus of claim 29, wherein laser diodes closer to the outlet face emit more power than laser diodes that are closer to the inlet face.
 31. Laser apparatus, comprising: a multi-mode, rectangular, self-imaging, waveguide, which has a core comprising a solid gain medium laminated between two sheets of cladding with exterior surfaces, said waveguide being sandwiched between two heat sinks, each of which has a heat sink surface that interfaces in contacting relation with one of the exterior surfaces of the cladding and that extends outwardly beyond the cladding to form a pump mounting surface; at least one laser diode pump light source mounted in thermally conductive relation to the heat sink surfaces and in a position to couple pump light into the core; and an optical system configured and positioned to direct a beam into the core of the waveguide with a desired spatial profile and to couple the beam out of the core after the beam has extracted pump energy from the core and at a re-imaging plane where, after having separated into multiple modes of propagation through the core, the beam re-phases into the desired spatial profile.
 32. The laser apparatus of claim 31, wherein the waveguide is a one-dimensional, rectangular, self-imaging, waveguide.
 33. The laser apparatus of claim 31, including a stacked array of laser diode pump light sources coupled optically to the waveguide.
 34. The laser apparatus of claim 31, wherein the heat sink comprises carbon—carbon composite.
 35. The laser apparatus of claim 31, wherein the heat sink comprises a copper micro/mini channel fluid heat sink.
 36. Laser apparatus, comprising: a multi-mode, rectangular, self-imaging, waveguide, which has a core comprising an unclad, solid gain medium and at least one flat side; a heat sink with at least one flat side positioned in thermally conductive relation to the flat side of the core; at least one laser diode pump light source mounted in position to couple pump light into the core; and an optical system configured and positioned to direct a beam into the core of the waveguide with a desired spatial profile and to couple the beam out of the core after the beam has extracted pump energy from the core and at a re-imaging plane, where, after having separated into multiple modes of propagation through the core, the beam re-phases into the desired spatial profile.
 37. The laser apparatus of claim 36, including an intervening, heat conductive, material positioned between the core and the heat sink, said intervening material having an index of refraction that is low enough not to interfere with waveguiding of light in the core. 